Mail Plus — using the built-in mail server on your Synology

Synology ships two packages for running your own mail server: Mail Server (free, somewhat dated) and Mail Plus Server (free for 5 users, license required beyond that). Both do IMAP, SMTP, webmail, spam filtering — a full mail server on your NAS.

Up front — plain talk: running your own mail server is considerably more demanding than “set up a NAS with a few clicks”. Unless you specifically want to control everything yourself or are after the learning experience, you’ll have an easier time with our mail hosting. There we take care of DKIM, SPF, DMARC, reputation, IP listings, updates — with Synology mail, all of that is on you.

If you still want to run mail on the Synology: here’s the setup.

When Mail Plus on the Synology makes sense

• You want everything on one device — NAS, cloud, mail.
• You only have 1–5 mailboxes and don’t need a mail migration from existing setups.
• You’re prepared to maintain DNS, firewall and reputation yourself.

When Mailcow with us is the better call

• You have more than 5 mailboxes.
• Mail migration from Microsoft 365 or Google is on the table.
• You don’t want to buy a license.
• DKIM/SPF/DMARC and sender reputation matter to you, but you don’t want to maintain them yourself.

Installing Mail Plus

1. Package Center → search for “Mail Plus Server” → Install.
2. Also install “Mail Plus” (the webmail frontend).
3. Open Mail Plus Server.

Setting up your domain

In Mail Plus Server under Domain → Add domain:

• Domain name: your mail domain (yourname.com, not the Synology subdomain).
• Mailboxes: define them per user.

For mail to work, you need DNS records at your domain provider.

DNS records

Mandatory records at your domain host:

Type	Name	Value
MX	@	nas.yourname.com. (priority 10)
A	nas	public IPv4 of your Synology
TXT	@	v=spf1 mx -all

Plus DKIM (see below) and DMARC.

Important: the MX record points to the A record of your Synology, not directly to an IP. So set the A record nas.yourname.com first, then the MX.

Enabling DKIM

DKIM cryptographically signs outgoing mail — without DKIM, many recipients drop your mail straight into spam.

In Mail Plus: Security → DKIM:

1. Enable DKIM.
2. Click Generate keys.
3. Mail Plus shows a TXT record you have to add to your DNS:

Name: default._domainkey
Type: TXT
Value: v=DKIM1; k=rsa; p=MIIBIjANBgkqhkiG9w0BAQEFAAOC...

Copy the value exactly and paste it into your DNS.

Setting up DMARC

Name: _dmarc
Type: TXT
Value: v=DMARC1; p=quarantine; rua=mailto:dmarc@yourname.com

We recommend setting it to quarantine (non-matching mail lands in spam) rather than reject (rejected outright) — reject can kill legitimate mail if your setup isn’t 100% right.

SSL certificate for mail connections

If you’ve set up Let’s Encrypt for your Synology (see Setting up a Let’s Encrypt certificate), use the same certificate entry for mail too.

In Control Panel → Security → Certificate → Configure, assign your LE certificate to the mail server service.

Testing the configuration

After DNS propagation (can take up to 24 hours):

1. Send a test mail to your Gmail address via the Mail Plus webmail or a mail client.
2. Open the mail in Gmail and use Show original.
3. Search the header for: spf=pass, dkim=pass, dmarc=pass. All three must say pass.

If one of them fails, the DNS setup isn’t quite right. Common causes:

• DKIM value mangled by whitespace while copying — verify against the original value.
• Broken SPF record — only a single SPF record per domain is allowed!
• DMARC record missing or wrong syntax.

Connecting a mail client

Server addresses for Outlook, Apple Mail, iPhone:

Service	Server	Port	Security
IMAP	nas.yourname.com	993	SSL/TLS
POP3	nas.yourname.com	995	SSL/TLS
SMTP	nas.yourname.com	587	STARTTLS

Auth: full mail address (user@yourname.com) and password.

Spam filter

Mail Plus has SpamAssassin built in. In Security → Anti-spam:

• Enable it.
• Spam score threshold: 5 (default) or 6 for stricter filtering.
• Action on spam: move to spam folder (don’t delete — otherwise false positives cause trouble).

Reputation and outgoing mail

This is where it gets tiring, which is why we often point people to Mailcow.

When you first start sending, receiving servers may file your mail as spam — your IP hasn’t built up any reputation yet. That changes after a few weeks of regularly sending mail that doesn’t get flagged as spam.

Useful tools:

• mxtoolbox.com: tests DNS records and blacklist status.
• mail-tester.com: you send a test mail there and get a score (10/10 is good).
• Google Postmaster Tools: shows how Gmail rates your domain.

We track the same things professionally for our Mailcow setups — problems get caught early there.

Don’t forget backups

Mail Plus data lives in /volume1/MailPlus. Include that folder in your Hyper Backup job (for example to our Hyper Backup Storage) — otherwise lost mail is lost for good.

Frequently asked questions

How many mailboxes can I use for free? Mail Plus Server: 5 users free. Beyond 5 you need a license, a one-time purchase of roughly €200–400 (depending on user count).

How does Mail Server differ from Mail Plus Server? Mail Plus is newer, has a webmail plugin (with calendar and contacts, SOGo-style), and better spam filters. Mail Server is the legacy version with fewer features. We clearly recommend Mail Plus.

Does ActiveSync work? No — Mail Plus can’t do Exchange ActiveSync. Push runs over IMAP IDLE, which many clients support (Apple Mail on the iPhone, however, only polls IMAP accounts at intervals). Our mail hosting also relies on IMAP plus CalDAV/CardDAV.

What if mail from a Synology IP doesn’t arrive? Some recipients block mail from DSL or consumer IPs. With us, your Synology has a business IP, so the problem is rarer. But: individual receiving mail servers have strict rules — often the only fix is building reputation or asking the recipient to whitelist your address.

Can I run Mailman or mailing lists? Not directly on a Synology. If you want Mailman, you need a Plesk vServer for that.

How does it compare to Mailcow at xaweho? Mailcow is considerably more capable and scales to more mailboxes, with CalDAV/CardDAV, MTA-STS and a modern spam system. Plus: we handle updates, IP reputation and blacklist problems. Mail Plus on the Synology is “functionally sufficient” for 1–5 mailboxes without high demands.

How do I migrate to Mailcow later? IMAP-to-IMAP migration with imapsync. Open a ticket, we’ll help.

What’s next

• Pointing your own domain at your Synology
• Setting up a Let’s Encrypt certificate
• Mailhosting — if you’d rather have it managed