How many characters are allowed to be in the subject line of Internet email? I had a scan of The RFC for email but could not see specifically how long it was allowed to be. I have a colleague that wants to programmatically validate for it.
If there is no formal limit, what is a good length in practice to suggest?
While RFC5322 defines a limit of 1000 (998 + CRLF) characters, it does so in the context of headers limited to ASCII characters only.
RFC 6532 explains how to handle multi-byte Unicode characters.
Section 3.4 ( Effects on Line Length Limits ) states:
Section 2.1.1 of [RFC5322] limits lines to 998 characters and recommends that the lines be restricted to only 78 characters. This specification changes the former limit to 998 octets. (Note that, in ASCII, octets and characters are effectively the same, but this is not true in UTF-8.) The 78-character limit remains defined in terms of characters, not octets, since it is intended to address display width issues, not line-length issues.
So for example, because you are limited to 998 octets, you can't have 998 smiley faces in your subject line as each emoji of this type is 4 octets.
Run php -a for an interactive terminal.
// Multi-byte string length:
var_export(mb_strlen("\u{0001F602}",'UTF-8'));
// 1
// ASCII string length:
var_export(strlen("\u{0001F602}"));
// 4
// ASCII substring of four octet character:
var_export(substr("\u{0001F602}",0,4));
// ''
// ASCI substring of four octet character truncated to 3 octets, mutating character:
var_export(substr("\u{0001F602}",0,3));
// '▒'
What's important is which mechanism you are using the send the email. Most modern libraries (i.e. System.Net.Mail) will hide the folding from you. You just put a very long email subject line in without (CR,LF,HTAB). If you start trying to do your own folding all bets are off. It will start reporting errors. So if you are having this issue just filter out the CR,LF,HTAB and let the library do the work for you. You can usually also set the encoding text type as a separate field. No need for iso encoding in the subject line.