Synology MailPlus Server setup: DNS, mailboxes and delivery checks

Applies to: Supported Synology NAS models with MailPlus Server. Package availability, account limits and menus depend on the model and installed version.

Plan the mail route before installation

A mail client composes and reads messages; an SMTP server transfers them; a delivery agent places received messages into a mailbox. Webmail opening successfully proves only part of that route.

Mail delivery crosses several systems. A working webmail screen alone does not prove that internet delivery is configured.

1. Confirm model support and recovery. Check that Package Center offers MailPlus Server for this NAS. Plan storage, updates, backups, retention and a recovery test. A single mail server needs an agreed outage and recovery plan.
2. Prepare your domain and connection. Use a domain whose DNS you can manage. For direct internet delivery, confirm a stable public IP, whether your ISP permits SMTP traffic and who controls reverse DNS. CGNAT or blocked TCP 25 can prevent direct inbound delivery; changing the NAS’s client-submission port does not fix that.
3. Map DNS correctly. An MX record points to a mail-server hostname, not directly to an IP address. That hostname’s A record points to the intended public IPv4 address. Arrange a matching PTR/reverse-DNS record with the IP provider for direct outbound delivery. Synology DDNS alone does not replace control of the mail domain’s DNS.
4. Prepare sender authentication. Publish an SPF policy covering every authorised sender, enable DKIM signing for the domain and publish its generated public key, then introduce an appropriate DMARC policy. Do not paste a generic TXT value without accounting for your existing Microsoft 365, marketing or other senders.

Install and activate a small test setup

1. Install MailPlus Server in Package Center. For a fresh system, follow Create a new mail system; an existing Mail Server or Exchange migration needs its own migration plan. Distinguish the email domain, such as example.com, from the server hostname, such as mail.example.com.
2. Select the correct account type, network interface and data volume. Sign in to the Synology Account for licence management. A compatible NAS includes five free active MailPlus accounts; additional active users need suitable licences and remain subject to that model’s limits.
3. Activate test users, install MailPlus and grant application access. Launch webmail from DSM. For another mail client, enable the required IMAP/POP3 service and configure the actual server hostname, ports, TLS and authentication shown by your MailPlus settings.
4. In Mail Delivery, require SMTP authentication for clients and use encrypted connections. Keep relay permissions limited. If the ISP or reputation constraints require an outbound relay, configure its supported authentication and TLS settings; do not create an open relay.

Earlier MailPlus wizard: use the fresh-system option only for a new deployment. Current package screens and migration options can differ.

Find where a message stops

Send between two local test users, then send to an external mailbox and reply from outside. Compare MailPlus delivery logs, the queue, any rejection code and the received message’s authentication results. If only external inbound mail fails, check public MX/A records and inbound SMTP reachability. If only outbound delivery fails, check the queue, ISP restrictions, PTR, authentication records and relay. An NTP/time check helps validate time and connectivity but does not prove every mail port works.

Confirm the result

Local, inbound and outbound tests pass, legitimate messages authenticate as intended, unauthorised relaying is rejected, and a backup can be restored before production migration.

If you need help

Include NAS model, DSM and MailPlus Server versions, direction of failure, exact SMTP error and time with time zone, and sanitised delivery-log excerpts. For account and diagnostic preparation use the Synology support-request checklist.

Updated 261108