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 http://ftp.sunet.se/pub/unix/mail/sendmail/KNOWNBUGS).
Best Answer
As per RFC 5321 a line during SMTP dialog must not exceed 1000 characters: https://www.rfc-editor.org/rfc/rfc5321#section-4.5.3.1.6
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.