This article applies to:

• MailMarshal/SEG Cloud

Symptoms:

• Sending a message through MailMarshal Cloud
• Message refused with SMTP response: 550 Mail refused by local domain enforcement policy

Causes:

• The sender (MAIL FROM) and recipient (RCPT TO) of the message are both external.
  • "External" means "not in domains managed through MailMarshal Cloud."
• Relaying of messages through MailMarshal Cloud between external addresses is not allowed.

Resolution:

For messages outbound from your servers, be sure the sender address is within your domains managed through MailMarshal Cloud.

Notes:

• One common reason for this issue is a Microsoft Exchange distribution list that has external contacts as members. With some Exchange versions, when a message is sent to the list by an external sender, Exchange attempts to deliver it to external recipients with the external user as the From address.
• To ensure delivery to distribution lists with external members:
  • For Exchange environments including Exchange Online, you can configure the main outbound connector so that it only sends messages through MailMarshal Cloud if it is "from" a local domain address.
    • Make sure there is an additional delivery option that sends the mail "from" external addresses directly, bypassing MailMarshal Cloud. 
    • Any mailing list messages would have been scanned inbound, so the threat/security risk from lack of outbound scanning is low.
  • Another option that works with any mailbox solution is to use mailing list management software. This type of software re-sends the messages with a valid local sender address. Many free solutions are available.

Trustwave MailMarshal Cloud KB article Q21050

Last Modified: August 19, 2018