Hi there Chris,
Dynamic IP addresses are fine, but they are no good if you are running a Mail Server, most of the big boys add these to their spam blacklists.
It's hard to say what's up.
In your SMTP settings who are you sending the mail to ie
SMTP Server : smtp.bt.com
We need to know who is running the mail server, your company or BT ?
Doubt it will be anything to do with the Router, unless one of your Sys admins has been 'doing' something to it ?!
Do you get 'normal' speeds downloading mail, accessing websites etc ?
Also remember that the Broadband connection that you have is a 'shared one'...
Depending on the package that you have it will be 'shared between 10 to 50 different BT customers' !
So if they are all using at the same time, everyone slows down...
Did you try the Broadband speed test ? What were the results ?
Sending emails takes a lot longer than receiving them, on a 2MBS ADSL line when you are downloading from a website, or recieving emails you get a maximum speed of 2MBS...but when uploading (ie sending emails) a normal 2MBS line will upload at around 256K, that's 1/8th of the download speed.
To fault find further, we need to know who / where the mail server is...
Have you checked the Broadband speed...
Have you tried a different computer on the network...
Have you tried using a different "Mail" app...
regards
Ric