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Tag: sysadmin

Courier MTA and “Invalid number of recipients”, 550 errors, and more.

Posted on November 17, 2015April 12, 2020 by wally

After months of frustration – having our outbound emails randomly fail for distribution groups (alias lists in Courier MTA terminology) – I found the cause. Our SMTP smarthost doesn’t allow multiple “RCPT TO:” commands in an email header. By default, Courier batches up all emails forwarded to the same domain name. There’s a switch to […]

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