There are three ways to connect a Synology NAS to the Internet: direct connection, PPPoE connection, or connection through a router. For detailed instructions on how to access a Synology NAS via the Internet, you can refer to this tutorial.

Having an external static IP address is crucial for a mail system. Although it is possible to run a mail system with a dynamic IP address, it is not as reliable as using a static one. We recommend registering an external static IP address for the mail system. For more information, contact your Internet service provider (ISP).

Configuring static IP/PPPoE

There are two ways to set up external static IP addresses on Synology NAS:

• PPPoE: Some Internet service providers (ISP) provide free static IP addresses; however, users must connect via PPPoE to retrieve a static IP address.

1. Sign in to DSM.
2. Go to Control Panel > Network.
3. At the Network Interface tab, select PPPoE and click the Edit button.
4. Set up the modem and network port.
5. Enter the username and password provided by your Internet service provider (ISP).

• Static IP address: If you already have a static IP address, you can enter it in Synology NAS.

1. Sign in to DSM.
2. Go to Control Panel > Network.
3. At the Network Interface tab, select a network port and click the Edit button.
4. Enter your static IP address.

A valid and registered domain name is required for clients to deliver emails to MailPlus

Server over the Internet. An email address has two parts. The part before @ is a username, and the one after @ indicates a domain name. For example, Alex's email address is "alex@example.com". His domain name is "example.com". To make sure an email address like "alex@example.com" works, you'll need to set up the MX record and A record to help emails reach MailPlus Server. You can configure these records on the DNS server of your domain provider.

MX record

MX record, or Mail Exchanger record specifies how the Internet should route your emails using Simple Mail Transfer Protocol (SMTP). Each MX record contains a hostname and a preference. A hostname guides emails to arrive at the right mail server. A preference points out the priority of multiple servers. The lower the preference number is, the higher the priority will be.

You can set up multiple MX records for a domain with multiple mail servers and assign each record a preference number. The primary server should have the lowest number, like zero, to ensure that this mail server responds to requests at first. When there is no response from the primary server, the Internet will try the other mail servers used for failover sequentially according to their preference numbers until one of them gives a response.

For example: if the email address is alex@example.com, you have to set up an MX record pointing to the mail server, which should receive emails on behalf of the domain example.com. Therefore, you should enter the domain you are editing in the Host field and the hostname of your MailPlus Server in the Points to field. The preference record that you assign to the primary server should be zero or closer to zero.

In this way, the MX lookup for example.com would return mail.example.com.

After the MX lookup finds the mail server, the Internet needs its IP address to locate the destination for mail delivery. That's why you need to set up A record for your mail server.

A record

A record, or Address record, points a domain or subdomain to the IP address of the host server. It allows the Internet to identify IP addresses when people use easy-to-remember domain names.

In the case of alex@example.com, mail.example.com is the subdomain of example.com, and the host server is the Synology NAS on which MailPlus Server is running.

The examples and the image are for demonstration purposes only. The DNS record interface that each provider offers may vary. If you have problems configuring DNS records, contact your domain provider.

Email authentication

It is highly recommended to set up SPF, DKIM, and DMARC for authentication, which guards against spam and identity theft. This configuration can reduce the chance of your emails being marked as spam by other email servers.

SPF

SPF (Sender Policy Framework) mechanism verifies the legitimacy of a sender's host. The SPF records of many domains are currently published in DNS and they provide the location of the hosts that are authorized to send emails on behalf of a domain. Therefore, when a host from a network delivers messages to MailPlus Server, the system will verify the SPF records of the sender's domain in DNS and determine if the host is authorized to send emails on behalf of this domain. If the SPF authentication fails, it will be categorized as fail or softfail depending on the SPF records, and the system will treat the two results differently.

For more information on configuring SPF, see this article.

DKIM

DKIM (DomainKeys Identified Mail) mechanism verifies a sender's identity using encryption methods to check if email content has been modified. With the DKIM mechanism, a sender's host will generate a set of the public key and private key and will publish the public key in DNS, while using the private key to create a digital signature to be affixed to emails. When the receiving host receives a message, it will check the public key for the sender's domain in DNS and use the public key to verify the signature, the sender identity, and whether the message has been modified or not.

For more information on configuring DKIM, see this article.

DMARC

DMARC (Domain-based Message Authentication, Reporting & Conformance) mechanism is based on SPF and DKIM verification methods. When the system receives a message, it will check if the sender passes the SPF and DKIM verification, thereby determining if the sender is fraudulent.

For more information on configuring DMARC, see this article.

Reverse DNS setup

The process of assigning specific DNS records to a domain name is known as forward DNS. This is what leads a domain name to the exact server. There is also a reverse process, known as reverse DNS.

What is reverse DNS?

Reverse DNS refers to translating numeric addresses of a website (i.e., the IP address) to the domain/hostname, as opposed to the forward DNS process which translates a domain or hostname to an IP address. Reverse DNS also refers to locating which domain name/host belongs to a given IP address; that is why this process is often referred to as reverse DNS lookup. When a domain name has a valid reverse DNS, it can be accessed via an IP address.

What does reverse DNS do?

Reverse DNS is one of the basic requirements for a mail system. It is often used as a spam filter to determine whether the IP address of an incoming message matches an authenticated domain name, and to block the message if it doesn't. If you don't set up reverse DNS for your mail server, messages sent from your mail server will be blocked by most major email providers. If you cannot set up reverse DNS by yourself and keep experiencing mail delivery problems, consider adding another SMTP server for mail delivery. We recommend that you use a well-known SMTP server to avoid being taken as a spammer when sending emails.

How to set up reverse DNS?

• Set up reverse DNS on your own host: Some ISPs may delegate a portion of the zone to users so that users can host their own reverse DNS. You can configure reverse DNS by determining PTR records in a DNS server. PTR records are managed by the entity that controls the IP address. It may be either your host or yourself if the host has delegated the reverse DNS for the IP space (containing one or multiple IP addresses) to you. A PTR record usually represents the IP address entered backward, followed by an in-addr.arpa entry.
• Set up reverse DNS with your ISP: The ISP or entity that owns your IP address is the only one who can add appropriate PTR records. You may have to contact them for reverse DNS configurations.

Once the installation is complete, you can start setting up MailPlus Server. In the section below, we will demonstrate how to configure basic SMTP (Simple Mail Transfer Protocol) settings. Note that the screenshots below are for reference only. Your settings may differ.

1. Go to Package Center to install MailPlus Server.
2. Launch MailPlus Server and select Create a new mail system if you want to set up a whole new mail system, and click Next to continue the setup. Otherwise, you can select Create a new mail system by migrating the data from previously installed Mail Server. Check this tutorial to see how to migrate Mail Server to MailPlus Server.
   
   
3. Enter your domain name and hostname (FQDN):
   • Domain name: A domain name is a location or an address where email messages are received. Check if the domain name matches the MX record in DNS settings.
   • Hostname (FQDN): A hostname is the address of your MailPlus Server. Check if the hostname matches the A record in DNS settings.



4. Modify the following settings according to your needs:
   • Account type: Select a user account type (local, LDAP, or domain users) that will be allowed to use MailPlus services.
   • Network interface: Select a LAN port used for MailPlus Server.
   • Volume: Select a volume on which MailPlus Server and its data will be stored.
5. Click Next to check the setup summary and click Apply to finish the settings.
6. After setting up MailPlus Server, you can Activate Accounts to allow specific users to use mail service. Note that activating more than five user accounts requires additional purchased licenses. For more information on the MailPlus license mechanism, refer to the MailPlus licensing page.

Access emails on Synology NAS with MailPlus

MailPlus is an add-on package that provides a web-based interface for client users to access and manage emails hosted on a Synology NAS.

Multiple POP3 accounts can be created in MailPlus, allowing users to fetch messages via other mail service providers (e.g., Mozilla Thunderbird, Gmail, and Microsoft Outlook).

Install MailPlus

1. Go to Package Center to install MailPlus.
   
2. Go to Control Panel > Application Privileges to allow target users or groups to access MailPlus.
   

1. There are two ways to launch the MailPlus login page:
   • Go to Main Menu > Synology MailPlus.
   • Access MailPlus via the Login Portal. Enter the name of the Synology NAS followed by "/mail" in the address bar of your web browser. For example, if the Synology NAS is called mydiskstation, enter mydiskstation/mail. Refer to this help article to see how to configure the login portal for applications.
     
2. Enter your DSM username and password to sign in.
3. If the settings of MailPlus Server have been configured before the installation of MailPlus, the SMTP settings of MailPlus Server will automatically appear at Settings > SMTP.
   

Access emails on Synology NAS with other email clients

Email accounts on a Synology NAS can be linked with various mail clients, such as Microsoft Outlook. In the example below, we'll show you how to use Outlook to access an email account hosted on a Synology NAS.

1. Launch MailPlus Server and go to the File tab.
   
2. Click Add Account.
   
3. Select Manual setup or additional server types.
4. Select POP or IMAP.
5. Fill in the following information:
   • Your Name: Enter the name you want to display in the From field.
   • Email Address: Enter your MailPlus email address.
   • Incoming mail server: Enter your MailPlus hostname (FQDN), e.g., mail.example.com.
   • Outgoing mail server (SMTP): Enter your MailPlus hostname (FQDN), e.g., mail.example.com.
   • User Name: Enter your MailPlus email address.
   • Password: Enter your MailPlus password.
6. Click More Settings... in the bottom right corner.
7. Go to the Advanced tab. Enter the port numbers for the incoming and outgoing servers.
8. Click Next and wait for Microsoft Outlook to set up your account.
9. Click Finish.
10. Once the setup is complete, your MailPlus account will appear on the left panel. Double-click the account to expand all the mailboxes.

Why can't I send or receive emails via webmail from MailPlus?

1. Check if the settings on your MailPlus such as SMTP, DNS, and MX are correct.
2. Check if the Internet settings of the Synology NAS are correct. Go to Control Panel > Regional Options. At the Time tab, tick Synchronize with NTP server and click the Update Now button to examine if the Internet settings are correct. If a result comes back successfully, the settings are correct.
   
3. Check if the port number on your router is correct.
4. Visit Spamhaus to check if your IP address is listed as a spammer. If so, remove your IP address from the block list on Spamhaus.

Why can't I send or receive emails via email clients?

1. Check if you have enabled IMAP and POP3.
2. Check if your username and password are correct.
3. Check if the settings on your MailPlus such as SMTP, DNS, and MX are correct.
4. Check if the Internet settings of the Synology NAS are correct. Go to Control Panel > Regional Options. At the Time tab, tick Synchronize with NTP server and click on the Update Now button to examine if the Internet settings are correct. If the result comes back successfully, the settings are correct.
5. Check if the port number on your router is correct.
6. Visit Spamhaus to check if your IP is listed as a spammer. If so, remove your IP from the block list on Spamhaus.

Why can't I receive emails sent from another mail server (e.g., Gmail)?

1. Make sure DNS settings are correctly configured. You need to point the MX and A records to your Synology NAS so that other mail servers can reach MailPlus Server.
2. Make sure the Synology NAS has a static IP address and is connected to the Internet, or your domain name points correctly to your dynamic IP.
3. If the Synology NAS is set behind a NAT firewall/router, make sure the port forwarding works properly. You can check whether or not the port forwarding is working properly by going to the CanYouSeeMe website and inputting the port 25.
4. If any, check the message in a returned mail to find the detailed reason for an error.

Why do I get rejected when sending emails to certain webmail accounts, like those of Gmail or Hotmail?

Many free email providers do a reverse DNS lookup to check the validity of a sender. If your reverse DNS lookup doesn't correspond to the sending domain name, your emails will be rejected. You can check with your ISP. Another possibility is that your IP address is listed on a spam block list. You can check this by visiting Spamhaus.