Those days we faced the problem that we recived a mail with approx 150 recipients.
Somewhere in the communication it seams that a mail address is broken by the asa.

On the Outside of the ASA you see following in the trace:

        Inside E-Mail Server (Blue) mail.example.com
        Outside E-Mail Server (Red) mail.asdf.com
        220-mail.example.com ESMTP Server [Wed, 18 Aug 2010 10:30:58 +0200]
        220-Ready to recycle your bits, but we don't want 
        220 your unsolicited or bulk e-mail (ie: spam) 
        EHLO mail.asdf.com 
        250-mail.example.com Hello mail.asdf.com [192.168.0.1] 
        250-SIZE 4194304
        250-PIPELINING 
        250-AUTH PLAIN LOGIN CRAM-MD5 NTLM 
        250-STARTTLS 
        250 HELP 
        MAIL FROM:<asdf@asdf.com> SIZE=42157 
        RCPT TO:<user1@example.com> 
        <output omited> 
        RCPT TO:<user20@example.com> 
        RCPT TO:<user21@ 
        250 OK 
        example.com> 
        RCPT TO:<user22@example.com> 
        RCPT TO:<user23@example.com> 
        RCPT TO:<user24@example.com>
        <output omited> 
        250 Accepted 
        <output omited> 
        250 Accepted 
        
            501 <user21@XXXXXXXXXXXXXX: '>' missing at end of address
          
        250 Accepted 
        250 Accepted
        250 Accepted

On the Inside of the ASA you see following in the trace:

        Inside E-Mail Server (Blue) mail.example.com
        Outside E-Mail Server (Red) mail.asdf.com
        220-mail.example.com ESMTP Server [Wed, 18 Aug 2010 10:30:58 +0200]
        220-Ready to recycle your bits, but we don't want
        220 your unsolicited or bulk e-mail (ie: spam)
        EHLO mail.asdf.com
        250-mail.example.com Hello mail.asdf.com [192.168.0.1]
        250-SIZE 4194304
        250-PIPELINING
        250-AUTH PLAIN LOGIN CRAM-MD5 NTLM
        250-STARTTLS
        250 HELP
        MAIL FROM:<asdf@asdf.com> SIZE=42157
        RCPT TO:<user1@example.com>
        <output omited> 
        RCPT TO:<user20@example.com>
        RCPT TO:<user21@ 250 OK
        XXXXXXXXXXXXXX
        RCPT TO:<user22@example.com>
        RCPT TO:<user23@example.com>
        RCPT TO:<user24@example.com>
        <output omited> 
        250 Accepted
        <output omited> 
        250 Accepted
        
            501 <user21@XXXXXXXXXXXXXX: '>' missing at end of address
          
        250 Accepted
        250 Accepted
        250 Accepted

This is a little bit strange so i will ask the Guys from Cisco if this is a known feature or a bug.