From: Frank Ellermann
Sent: Wednesday, December 01, 2004 4:49 PM
For longer addresses the resulting local part would have more
than 23 + 40 characters... BTW, where can I find this limit of
64 characters for id-left ?
Received: from quack.de.clara.net ([126.96.36.199])
by dagobert.de.clara.net with esmtp (Exim 3.16 #1)
7890(_at_)xyzzy(_dot_)claranet(_dot_)de; Wed, 01 Dec 2004 23:40:57 +0100
As you have just shown, your mail system accepts longer local-parts. During
some informal testing, I found that the great majority of systems had no
problem with 128 character local-parts. I believe there are a couple of
products that still have this limitation, but they are the exception and I
didn't actually locate any in my testing. I did verify that all of the
major MTA products accept much longer local-parts than 64-characters.
As for the source of the limitation, here is the relevant text from RFC2821:
188.8.131.52 Size limits and minimums
There are several objects that have required minimum/maximum sizes.
Every implementation MUST be able to receive objects of at least
these sizes. Objects larger than these sizes SHOULD be avoided when
possible. However, some Internet mail constructs such as encoded
X.400 addresses  will often require larger objects: clients MAY
attempt to transmit these, but MUST be prepared for a server to
reject them if they cannot be handled by it. To the maximum extent
possible, implementation techniques which impose no limits on the
length of these objects should be used.
The maximum total length of a user name or other local-part is 64
The maximum total length of a domain name or number is 255