Mailbox password not recognised

[#1]

Last week I created new mailboxes for family members. I was able to access these mailboxes via Webmail and from my email client (Outlook).
When I try to do the same thing today, I get an error message stating  'Unknown user or password incorrect ' from Webmail. The username and password were definitely entered correctly. Also, outlook cannot receive emails from these mailbox accounts stating 'There was a problem logging onto your mail server. Your User Name or Password was rejected.....'. The username and passwords configured in Outlook are definitely the same as the ones used to set up the mailbox.
Interestingly, this is a problem for 3 out of 4 mailboxes. One of them can be accessed successfully.
Any ideas what I'm doing wrong here?

[#2]

Is the mailbox that works the username one whereas those giving problems are additions?
Have you by any chance forgotten to use username+mailboxname as the login name for those additions?

[#3]

The mailbox that works is one of the username+mailboxname type. I have 3 others of the same type (with different mailboxnames) and they don't work - but they did when I originally set them up which is what I can't understand.
Yes, I am using the correct username+mailboxname login name on both webmail and outlook.
[My username mailbox also works fine and always has done.]
Thanks for replying.

[#4]

I had this problem myself sometime ago.... You need to raise a fault ticket, nothing you can do will fix this problem.
If it helps them my fault ID was 21933060. Took them 12 days to resolve my issue, perhaps they will be quicker with you.
Dan...

[#5]

Dan - thanks for the advice. Have raised a ticket and quoted your fault id.

[#6]

I've just tested a couple of your mailboxes and can log in to them without problems. Shout up if the problem carries on for you and I'll have another look.

[#7]

All working fine now. PlusNet support fixed it within a hour of raising the fault ticket