A User was getting the below bounceback sending to some servers:
This is the mail system at host out03.mx.trendmicro.eu.
"I'm sorry to have to inform you that your message could not
be delivered to one or more recipients. It's attached below.
For further assistance, please send mail to postmaster.
If you do so, please include this problem report. You can
delete your own text from the attached returned message.
The mail system
<email@example.com>: host mail.xx.com[x.x.x.x] said:
550-Verification failed for <firstname.lastname@example.org>
address 550 Sender verify failed (in reply to RCPT TO command) "
I initially thought it was a misconfigured SMTP smarthost so I Switched them over to Trend Micro hosted spam filtering (which includes outbound filtering via a smarthost) but they still get the same bounceback.
The server is SBS 2003 (which uses Exchange 2003), exchange is configured to forward all outbound e-mail to a smarthost (trend) which works fine for the most part, the virtual SMTP server is restricted to talking to only Trends servers if thats relevent.
I'm guessing theres something wrong with their domain/DNS but i've never seen this error, any ideas what to look for?