According to RFC2821 <http://tools.ietf.org/html/rfc2821>:
4.5.3 Sizes and Timeouts
184.108.40.206 Size limits and minimums
The maximum total length of a user name or other local-part is 64
We are experiencing a problem with an important client that uses Earthwave <http://www.earthwave.com.au/> to look after their mail. Their mail gateway blocks our CiviMail mails because of this RFC violation (specifically the Return-Path bounce VERP address used in the MAIL FROM). Presumably there are other mail providers out there that do the same.
An example VERP email address CiviMail generates would be:
It seems Moodle use VERP and complies with RFC (keeping the local part <= 64 characters):
Ideally the VERP emails would be formatted in a way to ensure they complied with this RFC policy.