New broadband account today, email hates either my password or username

[#1]

I have set up Thunderbird as per the guide. and also tried logging into webmail, but both ways I am told my password is not accepted.  Just in case I had forgotten how to spell the password, I went to my account and changed my password - it happily accepted the "wrong" password for the Current Password box.  But mail will not accept my new password either.  Everything else about plusnet likes my password. (NB I know the case has to be correct, before anyone suggests checking that :D)

[#2]

Did you choose to take a Plusnet email address during signup as from our side it doesn't seem so.
If not, you need to go to the link below and choose your email address.
https://portal.plus.net/my.html?action=add_mailbox
Let us know if there are any issues.

[#3]

Well I didn't think I need a plusnet address as I have others for incoming, I was just assuming I would use the SMTP for sending as usual?  Is that not the way it works now?
ETA ok sorted - I am just using the SMTP from one of my other accounts

[#4]

I don't think you need to use the PN servers for sending, assuming you have others available.
Unless you've blocked ports on the PN firewall.
I helped someone (a PN customer) just yesterday set up Outlook for an email account hosted elsewhere and it worked fine using their servers for send & receive.

[#5]

Yep, I edited post above.  I had until yesterday been relaying through my previous ISP which of course I don't have any more, and had made an assumption based on that, so I have just changed to using one of the other accounts.