You can turn your Synology NAS into a mail server, allowing Synology NAS users to receive and deliver mail messages.
This article will give you basic information on how Synology MailPlus Server works and what you need for the setup of MailPlus Server. If you need step-by-step instructions, please refer to this article.
You need a domain to set up an email address. An email address has two parts. The part before @ is the username, and the part after @ indicates the domain name.
You need to buy and register a domain name from your ISP or third-party domain providers and set it up in MailPlus Server. We do not recommend using Synology DDNS because running a mail server on dynamic DNS may cause undelivered emails.
MailPlus Server can handle emails for multiple domains, so you can add more than one domain to your MailPlus Server.
DNS records are necessary for a mail server to receive and send emails. The right DNS records also help designate your MailPlus Server as a legitimate mail server.
Below is the basic information on some types of DNS records. For detailed instructions on DNS setup, refer to this article.
Mail exchanger record (MX record) indicates where emails should be routed to. You have to set up an MX record pointing to your mail server so that the Internet will know where to send your emails. A domain can have more than one mail server, so you can set up multiple MX records for a domain with multiple mail servers and further configure MX record priorities. The lower the preference number is, the higher the priority will be. For example, to make sure ann@example.com works, you have to set up an MX record pointing to the mail server, which should receive emails on behalf of the domain "example.com".
Address records (A records) point a domain or subdomain to an IP address. You have to point the A record to the IP address of your Synology NAS if you want to receive emails on your MailPlus Server.
Reverse DNS helps a mail server find out the hostname or domain behind an IP address. Many mail servers use reverse DNS to determine whether the sender's domain name is associated with spammers. Therefore, emails sent from an IP address that does not have reverse DNS are likely to be rejected. Please note that the reverse DNS for your mail server must match the hostname (fully qualified domain name, FQDN) of your MailPlus Server. If MailPlus Server's reverse DNS and hostname do not match, emails from your server may be treated as spam.
Once your email domain and DNS records are ready, you can follow the setup wizard when launching MailPlus Server for the first time. However, the easy flow often fails since it is quite difficult to know the name and type of a mail server. If you wish to know more about a mail server, please continue to read the following paragraphs.
MailPlus Server offers two solutions: a single-node configuration or a high-availability configuration. In the single-node configuration, only one Synology NAS is required to run the mail service, and it is the easiest way to set up and get going. In the high-availability configuration, two Synology NAS form a high-availability cluster to ensure uninterrupted service when an unexpected error strikes.
SMTP servers are the real workhorses that send thousands of messages every day. However, most ISPs block or reject emails from unknown domains. If you cannot send emails directly from your domain, you may have to send them through a well-known SMTP server. You can configure the SMTP relay server on MailPlus Server.
You can also map multiple domain names to MailPlus Server (e.g., example.com, example.com.tw, and example.com.us) to let users receive emails sent to various addresses (e.g., user@example.com, user@example.com.tw, or user@example.com.us).
IMAP or POP3 servers handle connections from incoming IMAP or POP3 clients such as Mozilla Thunderbird, Microsoft Outlook, and Apple Mail. IMAP and POP3 servers grant clients access to the emails saved on the mail server.
Most mail servers and clients support both IMAP and POP3. POP3 applies to users with a single device while IMAP is dedicated to users with multiple devices. POP3 clients download emails from the server and save them locally. IMAP clients modify emails on the server, which will be mirrored to all the IMAP client mailboxes; therefore, all the changes made to an email will be synchronized across multiple devices.
To secure the data transfer via IMAP and POP3 connection, we recommend that you use SSL and TLS encryption.
The full-text search feature automatically indexes emails to improve search performance. Indexing emails with Chinese, Japanese, and Korean characters requires additional computing resources. You may evaluate the needs and determine whether to enable full-text search for Chinese, Japanese, and Korean emails. You can also specify the users or groups to use the full-text search feature to avoid overconsuming system resources.
See the table below to check and set up port forwarding if the network address translation (NAT) is required.
| Protocols | SMTP | POP3 | IMAP | IMAPS | POP3S |
|---|---|---|---|---|---|
| Port | 25 | 110 | 143 | 993 | 995 |
| Note | IMAPS: IMAP over SSL/TLS POP3S: POP3 over SSL/TLS |
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If you have set up a mail cluster, when any of the SMTP/SMTPS/SMTPS-TLS ports on a server are changed, you must go to DSM > Control Panel > Security > Firewall on the other server to update the firewall rules about the ports related to MailPlus Server.
Likewise, firewall rules will not be automatically synced between two servers in a cluster. When a new server joins the cluster, you must go to DSM > Control Panel > Security > Firewall on the newly added server to manually set the corresponding firewall rules.