Very slow email connection on one mailserver only

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!