We have a mixed environment of exchange 2003 / 2007. We are having a strange issue. when i try sending email with message size >10MB from exchange 2003 to exchange 2007, it fails and i get NDR with 5.2.3 status. however, when i send the same email the other way, it gets delivered. It only happens with messages >10MB. i have changed the setting of exchange 2007 at organization level, receive connector level and user mailbox level to receive emails upto 30 MB in size. Also, i have changed the settings on exchange 2003 at administrative group > Protocols >SMTP virtual server level to allow 30 MB of email to go through. however, sending emails of size > 10 MB from exchange 2003 to 2007 still fails. i have rebooted the information store and still its the same. i enabled diagnostic logging on exchange 2003 and still could not find anything concrete. I have observed that email never reaches exchange 2007 server. Another observation, mails >10 MB flows without issues within exchange 2003 mailboxes and so is for exchange 2007. Please let me know what am i missing.