I have iRedMail on Debian Squezze, and it was upgraded for 7.3.
For 3  weeks I have serious problems, I read all the topics with similar errors but I haven't fond the solution yet.
For www@mydomain.hu I always get similar e-mails with this error:

"Transcript of session follows.
Out: 220 Microsoft Exchange 11.21
In:  EHLO mayor.hu
Out: 250-mail.szistvan.hu
Out: 250-PIPELINING
Out: 250-SIZE 15728640
Out: 250-ETRN
Out: 250-STARTTLS
Out: 250-AUTH PLAIN LOGIN
Out: 250-AUTH=PLAIN LOGIN
Out: 250-ENHANCEDSTATUSCODES
Out: 250-8BITMIME
Out: 250 DSN
In:  MAIL FROM:<lista-bounces@mayor.hu> SIZE=3515
Out: 250 2.1.0 Ok
In:  RCPT TO:<balintis@szistvan.hu> ORCPT=rfc822;balintis@szistvan.hu
Out: 451 4.3.0 <lista-bounces@mayor.hu>: Temporary lookup failure
In:  DATA
Out: 554 5.5.1 Error: no valid recipients
In:  RSET
Out: 250 2.0.0 Ok
In:  RSET
Out: 250 2.0.0 Ok
In:  MAIL FROM:<senders-mail@mayor.hu> SIZE=3746
Out: 250 2.1.0 Ok
In:  RCPT TO:<recipient-mail@szistvan.hu> ORCPT=rfc822;recipient-mail@szistvan.hu
Out: 451 4.3.0 <senders@mayor.hu>: Temporary lookup failure
In:  DATA
Out: 554 5.5.1 Error: no valid recipients
In:  RSET
Out: 250 2.0.0 Ok
In:  RSET
Out: 250 2.0.0 Ok
I increased the connections in /etc/mysql/my.cnf as was suggested (300 connections permitted). The recipient is VALID. This problem occurs randomly. Sometimes the recipient gets the mail, sometimes he is bounced as "no valid recipients". Sometimes from domain.xx he gets the mail, sometimes from domain2.xx he never gets the mail.
I've looking for the solution in this forum and Google for 3-4 weeks, but my problem was not solved yet.
Thanks for Your help. (Sorry for my week English)
Istvan Balint