Very slow email connection on one mailserver only

Question

I will try and keep it short but it is complex (to me!):
I have suddenly, since Friday 2 March, experienced very slow connection to one of my ISP's mailservers. Emails without attachments work ok'ish (a little "hesitantly"), emails with attachments of moderate sizes (eg. +/- 500 KB) download/upload extremely slowly and most of the time fail. I have asked my ISP, who are generally helpful, and they have tested the mailserver and can find nothing wrong. So, here we go:
The problem is both for ethernet and wireless.
The problem is for one account only. I have a different account with the same ISP, but on a different mailserver, which works absolutely fine.
Two different Macs have the same problem. The other PCs with different ISP's accounts work fine.
Router is working fine with good internet connection and email via one of the two accounts works fine.
It is not a problem on my computer (since it is the same issue for two machines). Both run OSX 10.6.8
I have tried both Entourage and Outlook on both machines; same problem everywhere - one account works, the other doesn't (properly).
I have tried  with my iPhone, using the same wireless connection and same account details, and it downloads emails with attachments speedily - emails that failed to download on the MBP/Outlook.
I have tried one of the machines (a MacBook Pro) at home with a different wireless connection but to the same mailserver and it works fine there!
This happened last Friday and no settings have been changed locally for months.
My conclusion is (but may be wrong):
The router is fine
The macs are fine
The email clients are fine
General connection to internet and mail is fine
The mailserver is probably fine
But a connection between one mailserver account and the two macs has suddenly gone wrong.
It is almost as if there is a setting on either the mailserver saying "I don't like that connection to that place, I will slow it down." or somewhere in this place saying "I don't like that account on that mailserver, I will slow it down."
The question then is:
Which network setting is so specific that it can be this selective? And why did it suddenly happen?
I don't seem to get anywhere, so I'm trying the communities. Any help much appreciated!

Answer 1

Update: After two weeks of scratching my head trying to find any local settings (computer or router) that were specific enough and after really not doing anything at all, normal service resumed suddenly. I have now had a good short week without any disruption or speed issues. The problem that was is still a mystery, though.