As a follow up I have the 550 problem fixed.
After a lot more testing I found this problem was only happening on sites where the mail.?.com and www.?.com were hosted on different IP's.
Yup. Just as I said, IMail was going to the A record rather than the MX record. Since most sites have example.com point to the same server as www.example.com, your finding makes sense.
Sometimes our SMTP server would connect to mail.?.com in which case everything worked,
and sometimes it connected to www.?.com in which case it would fail but still respond with an SMTP mail response as you saw.
Close.
The domain "?.com" has an MX record that points to mail.?.com. IMail should go there. However, on your server, IMail is occasionally going to just "?.com" (which coincidentally happens to be the same IP as www.?.com).
I looked back at the IMail config and found specifically for the SMTP server there were hard coded DNS server addresses entered that reflected our previous network DNS server addresses which was replaced 1.5 months ago.
IMail is doing some unusual things now with the DNS server settings. For example, if it is left blank, IMail will display in IMail Administrator the exact same thing as if you entered the Windows DNS settings. It may be that it is defaulting to the Windows DNS settings if the ones you have entered aren't reachable -- and perhaps that is exactly when IMail accidentally goes to the A record rather than the MX record.
-Scott
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