Password for email accounts

[#1]

Following the email service going offline a few days ago and causing all logins to prompt for a password, I am still being prompted for a password to access my postmaster account. When I enter the password, I get a message
"mailserver mail.plus.net responded: login failed"
The other personal accounts work OK (with the same password), just not the postmaster@<account>.plus.com
I'm using Thunderbird (v2.0.0.9) as the email client. I don't know whether it's a problem with the Plusnet email service or Thunderbird.
If it's the latter, any ideas gratefully received.
Cheers

[#2]

Is the postmaster account a separate mailbox?
Try logging into it using webmail. Remember, if its a separate mailbox the login will be username+postmaster.
If that fails, reset the password for the mailbox on the portal and try webmail again. If that works check your email client is set correctly with the new password.

[#3]

Also, I've been known to forget that separate mailboxes have separate passwords

[#4]

Have you tried logging in via webmail to the affected mailboxes? If you can log into webmail then I'd suggest recreating your profile in Thunderbird (adding it again and removing the exisiting one).
You could also try telnetting to the mail server and seeing if that allows authentication.
To perform a telnet in windows:-
Open up command prompt
Type in

telnet mail.plus.net 110

and press enter
You should get an acknowledgement similar to this

+OK AVG POP3 Proxy Server 7.5.510/7.5.516 [269.19.11/1242]

Type

user username

and press enter
Type

pass password

and press enter
Replace username and password with your details
You should get a message similar to this:-

+OK logged in.

If this works then it shows that authentication to your mailbox is ok, and I'd suggest recreating your profile in Thunderbird.