Maximum X-Headers Email Size

Question

We are looking at attaching some metadata to the X-headers of email messages. These emails are intended for use by internal systems and will be hosted on the Exchange server.

Is there a maximum size for the data we can store in X-Header?

Are there any restrictions, such as special characters, that I should be aware of?

Answer 1

US ASCII characters only.

This is defined by RFC 822 .

There is no restriction on the length of the header body in the standard, although there is a restriction on the length of the string, which imposes a restriction on the length of the header name.

There are two restrictions that this specification places on the number of characters in a string. Each character string MUST be no more than 998 characters, and MUST be no more than 78 characters, excluding CRLF.

You can, however, expand the body of the header beyond the same “fold” line. Then the receiver "turns" the line.

The expanded header field has no length limit and therefore can be indefinitely long.

The title name cannot be collapsed, so the title name cannot be longer than the limit.

Please note that even if the standards do not have restrictions on the total length of the header body, actual implementations may impose artificial restrictions.