Server Email Blocking <?>

I recently signed up a new client to my dedicated server - The minute they switched over to my server, it seems that all hell broke loose. (I'm going to refer to them as "Company A")

Company A called me up and said that one of their employees was getting a huge amount of SPAM and that after a day or two, they were having issues with their E-mail.

I looked at my logs and it showed something unusual-

LOGIN FAILED, user=myclientuser@companya.com, ip=[::ffff:XX.XXX.170.47]: 110 Time(s)

When I explained this to Company A, they ran some virus checks on their computers and 3 out of 5 computers had viruses on them.

They claim to have fixed the viruses but now, they cannot send e-mails to specific clients.

I checked their I.P. against blacklists and they are using Comcast cable internet at their location and I cleared their only blacklisting (spamhaus.org).

I'm still getting calls that Company A cannot e-mail a few of their clients and just to make sure it's not JUST them, I tried to send a test e-mail to the same clients as Company A.

The e-mails from me were rejected due to time-out.

HERE IS MY QUESTION:

Is this an issue on MY end that must be taken care of *OR* is it due to the fact that they had viruses on their computers and now they are blocked because the virus tried to attack everybody in THEIR e-mail address book?

None of my other clients are complaining of e-mail issues or that e-mails are getting kicked back. Just Company A.