What is the email subject length limit? -

How many characters are allowed in the subject line of internet email? I was scanned but could not specifically see how long it was allowed. I have a colleague who wants to programmatically valid for this.

If there is no formal limit, then what is a good length in practice to give suggestions? Cheers,

See, start section 2.1.1.

There are two rows that put this standard on the number of characters in a row. Each line of characters will not exceed 998 characters, and excluding CRLF, should not exceed 78 characters.

As RFC later tells you, you can work around this limit (not what you want) by adding the subject to multiple rows.

Each header field is logically a line of characters in the field names, colon, and field body. However for convenience, and to deal with the 998/78 character limitations per line, the regional part of the header field can be divided into a single row representation; It is called "fold", the general rule is that wherever a standard white space (WSP characters only) allows to fold, a CRLF can be inserted before any WSP, for example, header field:

  Topic: This is a test  

can be presented as:

  Subject: This is an exam  

Recommendation of more than 78 characters in the subject heading is not appropriate. Nobody wants to scroll to see the whole subject line, and can cut something important right on it.