One user of mine has been having sporadic email delays for the last two weeks and I cannot figure out why. We called our email Gateway provider (Dyndns) and they examined a few of the email headers I've forwarded them and after looking at them they said that it looks like the emails are getting queued up because our server was not accepting the IP it was coming from and it kept getting deferred until it accepts it from one of the IP's our server "likes".
To try to resolve that, they recommended that I whitelist their IP range on both the server and in out firewall which was done last week yet we are still seeing delays on some emails. After speaking with them several times they said there is not much else they can troubleshoot as it seems to be the connection between their server and ours.
Besides for whitelisting, what else can I do to troubleshoot this issue. Is there any logging I can enable on our SBS server?