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Does Exchange 2010 impose a maximum length for individual headers? If so, what is it?

Let's say a user sends an email to 200 recipients, resulting in a TO: field having 200 email addresses (very bad idea I know). Or let's say an upstream content filter adds an X-header that has a lot of information.

At what point would Exchange reject the message. Would it reject the message?

The reason I ask is because I've seen other MTAs have issues with large headers and am curious about Exchange. For example, by default, SendMail milter communication fails if a single header is larger than 64K (as per

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It's beside the point, but Exchange lets you limit the number of recipients of a message. – Ward Dec 15 '11 at 1:43

As per RFC 5321 a line during SMTP dialog must not exceed 1000 characters:

This is also true for email header lines according to RFC2822 ff. The same RFC introduces multi-line headers to get around that limit. Therein there is no limit for a single header.

So if there is a limit, then it is an Exchage "problem". The best would be to test for yourself and give feedback.

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There has to be some limit, even it's it simply the disk space available to store a header while the e-mail is in transit. He just wants to know if there's something else. – Chris S Dec 16 '11 at 19:29
@ChrisS Sure there is a limit. But who sends mails with 2GB of data in one header? Even if it is 1MB it is a ridiculous big header. – mailq Dec 17 '11 at 22:17
@mailq Here's a use case example, lets say an email was sent to large distribution list. For auditing purposes, it's important to know who was exactly a part of that group email address at the time the message was sent. So the archiving solution breaks apart and displays all the recipients who belong to that group address at the time. This results in a massive recipient header. – Mike B Jan 5 '12 at 1:02
up vote 1 down vote accepted

Found the answer here:

By default, Microsoft imposes a limit of 64 KB as well.

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