SMTP Auth -- Comcast, Verizon, and Sorbs -- Problems
After years of allowing POP before SMTP, and a few years of both SMTP Auth and POP before SMTP, we made the switch to SMTP Auth only.
We use a number of RBL providers to help prevent spam.
dul.dnsbl.sorbs.net was one of the RBL's we have been using successfully for some time.
Most of our clients made the switch over without any problems.
But we had approximately 10 clients using either Comcast or Verizon that had trouble.
While their settings were perfect, when they tried to send email, they were being blocked by the dul.dnsbl.sorbs.net RBL as if their personal computer was trying to send out via their ISP IP rather than directly connect to our mail server.
The closest explanation to this problem was found on http://service.futurequest.net/?_a=k...details&_i=165
Specifically "your ISP may be intercepting connections on this port. This is similar to blocking the port, but instead of refusing the connection, your ISPs servers will jump in and act as your SMTP server."
The moment we removed dul.dnsbl.sorbs.net from the RBL's we utilize, the problem went away.
Customers of other ISP's did not experience this problem, even with dul.dnsbl.sorbs.net turned on.
Does anyone know if SMTP Auth and dul.dnsbl.sorbs.net don't get a long in general?
OR
Can put some technical details into what ComCast or Verizon is doing differently than other ISP's that would cause this to happen?
NOTE: POP before SMTP with dul.dnsbl.sorbs.net using ComCast and Verizon work. The issue only showed up when using only SMTP Auth.
Thank you.
