"Line too long" Error Explained

What Does "Line too long" Mean?

Occasionally, some emails may hard-bounce with the following error:

Line too long (see RFC5321 4.5.3.1.6) - ImprovMX

This indicates that the email failed to be processed by ImprovMX due to not being RFC compliant as per RFC5321 4.5.3.1.6:

"The maximum total length of a text line including the <CRLF> is 1000 octets (not counting the leading dot duplicated for transparency). This number may be increased by the use of SMTP Service Extensions."

To prevent this error, the sender of this email will want to double check that their email and all headers are correctly formatted.

How We Handle This Error

Some other email providers may not enforce this rule. However, as an email forwarding service, we are ultimately responsible for all emails that we forward. Even if we were to accept the email, there is a risk that the next hop will refuse the email due to it not being RFC compliant, which could harm our sender/IP reputation.

It is important that we avoid anything that could impact our sender/IP reputation. Enforcing this rule is one of the ways we achieve that.