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I am occasionally receiving error messages from mail servers with the above message. It seems that at least one line in the e-mail has more than 999 characters and therefore the e-mail is bounced.

Is this "by design"? in an RFC? Or some weird pseudo-spam-filter? Or just a bad mailserver?

I googled around a bit but did not find a competent answer. Hopefully one of you guys can enlighten me.

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up vote 1 down vote accepted

It can be a setting in firewall or MS Exchange to attempt to defeat spam. It is related to the number of characters in a single line of the message headers. In the past, spammers might have sent email to hundreds of addresses in the To or Cc line of the email, and this restriction was created to filter out such email.

There are also limits in message body. Postfix defaults to 990 length limit, but it does not bounce email based on this. It will insert newlines to prevent the line from being longer.

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