cannot use imap

[#1]

Just today I am having problems that I cannot login to Plusnet email using Thunderbird and imap.  smpt failing as well.
Is there a server problem?
I installed a trial of Kaspersky yesterday so that could be related, but turning both it and its Firewall off doesn't seem to make any difference.
I can get on fine with webmail, hardly ever use plusnet mail anyway, and what little mail I get is also forwarded to my gmail, so it isn't a significant issue, but I'd like to be able to fix it.

[#2]

Try turning off Anti Spam and Mail Anti Virus in Kapersky and try again.

[#3]

Thanks, that does resolve the issue. 
I didn't get the issue on gmail and other real email services.  It seems to derive from the fact that PlusNet only provides toy security on its IMAP (or POP) connections.  At least the help pages all seem to indicate that.