text line The maximum total length of a text line including the <CRLF> is 1000 characters (not counting the leading dot duplicated for transparency). This number may be increased by the use of SMTP Service Extensions.
This is an issue you cannot avoid consulting your site support group to resolve. Possible problems include the output class you are using, the writer name you are using, the format of the email, SMTP issues, TCP/IP issues, having line numbers in your email (which will keep the email from being sent) -- and that's just the start. Your site support group can help you identify where the problem is and how to resolve it.