Hi Forensic, thanks for your feedback.
We haven't encountered such problems so far, lets find out where the problem lies.
As UK2.net apparently doesn't provide free accounts, we'd appreciate some further help from you.
Please turn off 'Scan secured connections' option in avast! Mail Shield settings. Then set your UK2 account in Apple Mail to use unencrypted connection. Don't worry, avast! encrypts the connection all the same.
Did it help?
Anyway, please send us original connection settings you're using for UK2 account, including server, port and authentication type.
Thanks for your cooperation and looking forward hearing from you.