Start/notes.ini Parameters/SMTPMaxRecipientsPerMessage

SMTPMaxRecipientsPerMessage

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Parameter: SMTPMaxRecipientsPerMessage
Short description: Maximum number of recipients per inbound SMTP message – prevents mass / spam mails with oversized recipient lists.

Profile

Parameter
SMTPMaxRecipientsPerMessage
Category
Mail / Router
Component
Server
Available since
9.0
Supported versions
9.0.1, 10.0, 11.0, 12.0, 14.0, 14.5, 14.5.1
GUI equivalent
Configuration document
Possible values
Integer, e.g. 100, 500; 0 = unlimited

Description

At the SMTP level, a single message can address many recipients via RCPT TO. Spammers and mass mailers exploit this to push mails with hundreds of recipients through in a single transaction. SMTPMaxRecipientsPerMessage draws a hard line here – above the limit, further RCPT TO commands are answered by the listener with a permanent error. For typical business communication, values between 100 and 500 are easily sufficient.

Example configuration

SMTPMaxRecipientsPerMessage=200

Notes & pitfalls

  • Values that are too low can affect legitimate newsletters / distribution lists – measure what actually occurs first.
  • Takes effect after restarting the SMTP task.
  • Complementary to SMTPMaxForRecipients (per-sender limit) and antispam heuristics.
  • Use 0 = unlimited only in closed setups.