Configure the spam filter (SpamAssassin) in Plesk

Plesk uses SpamAssassin to score every incoming message. Messages above a threshold are marked, moved to a Spam folder or deleted. Good spam settings keep inboxes clean and also stop spam from being forwarded elsewhere. This guide covers both the administrator's server settings and each mailbox's own settings.

Before you start

• The SpamAssassin component is installed. Administrators check Tools & Settings → Updates → Add/Remove Components → Mail hosting features → SpamAssassin.
• An existing mailbox in Plesk.

Server-wide settings (administrator)

1. Go to Tools & Settings → Mail → Spam Filter (in some versions Mail Server Settings → Spam Filter).
2. Tick Switch on server-wide SpamAssassin spam filtering.
3. Tick Apply individual settings to spam filtering so users can tune their own mailboxes.
4. Set the Maximum number of worker processes according to RAM; 5 suits most VPS servers.
5. Set the default Spam score, typically 7 to start.
6. Click OK.

Administrators should also enable greylisting and DNS blackhole lists (for example zen.spamhaus.org) under Mail Server Settings; they stop much spam before SpamAssassin even sees it.

Mailbox settings

1. Go to Mail and click the address.
2. Open the Spam Filter tab.
3. Tick Switch on spam filtering for this email address.
4. Set The score that a message must receive to qualify as spam. Lower is stricter: 5 is aggressive, 7 is balanced.
5. Choose what to do with spam:
   • Move spam to the Spam folder – recommended.
   • Mark spam by adding the following text to the message subject – useful with POP3 clients, for example ***SPAM***.
   • Delete spam – avoid; legitimate mail can be lost.
6. Click OK.

Allow and block lists

On the same tab open Show more settings (or the Black List and White List sections):

• Add trusted senders to the allow list, for example *@yourbank.com.
• Add persistent spammers to the block list, for example *@spammer.example.

Wildcards such as *@domain.com are supported. Do not allow-list your own domain; spammers forge it.

Train the filter

SpamAssassin's Bayesian filter learns from examples. When users move messages to or from the Spam folder in Roundcube or an IMAP client, Plesk can learn from them if training is enabled by the administrator. Consistent training for a few weeks improves accuracy noticeably.

Troubleshooting

• Good mail in Spam: raise the score a little, add the sender to the allow list and ask the sender to fix SPF and DKIM.
• Spam tab missing: server-wide filtering or individual settings are disabled.
• Spam still in Inbox: check message headers for X-Spam-Status; if absent, the message was not scanned, often because the mailbox filter is off.
• High server load: reduce worker processes, or enable greylisting and RBLs to filter earlier.

Next steps

• Set up forwarding and auto-replies.
• Configure SPF, DKIM and DMARC.
• Create email accounts in Plesk.