Bizarre interaction between plusnet firewall settings and mail.btinternet.com

[#1]

Something I observed t'other day:
I have a btinternet premium mail account from donkey's years ago which is quite useful. I use the main account, and I have a number of sub-accounts, one of which is used by 'er indoors.
With the plusnet firewall settings on 'low', everything works fine. If I change the plusnet firewall settings to 'high', the main email account continues to work fine. The sub accounts can still recieve mail, but can't send anything - mail.btinternet.com demands the sub-account password and doesn't recognise it correctly.
Setting the plusnet firewall settings back to 'low' cures the problem .
My guess is that the 'high' firewall settings interfere in some way with SMTP authentication, but I don't understand why only the sub-account is affected.
It's not a problem, I can get by with the 'low' settings, but I'm curious to know if anyone else is seeing a similar problem and what the cause might be.
Kind regards,
Pete

[#2]

I would imagine the firewall is restricting the SMTP traffic to PN's servers only, this would stop Mailbots sending out unsolicited mail from your connection, or that would be my gut assumption as to what is happening.
Scott

[#3]

Don't suppose you know what port your outbound mail's going over? Is there a possibility that you have a local antivirus client proxying outbound mail through a different port?
AFAIK the firewall won't restrict any outbound traffic, it's all inbound.
Presumably you have a PlusNet subscription account? Out of interest, can you use SMTP auth with your PlusNet mail account?

[#4]

AVG is good for messing up email accounts, had that happen a few times