email: fasthosts problem, or plusnet?

[#1]

I have a problem with my fasthosts POP3 account; every so often it stops allowing email to be sent, but will still allow it to come in and download. Thing is, though, the error message flags up plusnet - see below:
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Cannot send the message using 'smtp.fasthosts.co.uk'
The server response was: <87.1xx.1xx.x.plusnet.thn-ag1.dyn.plus.net[87.1xx.1xx.x]>: Client host rejected: Access denied
Use the pop-up menu below to try a different outgoing mail server.  All messages will use this server until you quit Mail or change your network settings.
Message from: Mark Murison <info@bla-bla-blahhhhhh>
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The plusnet account works fine - I just don't understand why its mentioned in the eror message...could plusnet have a problem with my fasthosts account, or something? anyone got any easy to follow suggestions, please? markoos

[#2]

Seems to me that Fasthosts are rejecting access to their SMTP server from your dynamic IP address. Doesn't explain the intermittent nature of the problem though (although your address will change each time you connect). Perhaps it's overzealous spam filtering on their outgoing servers although not sure why they'd need this if you have to use a username and password to send through them.
Change the outgoing server of your fasthosts account to relay.plus.net in your mail client. That should fix it.

[#3]

Okay, I'll give it a try - thanks for the fast response!

[#4]

Okay, tried that - sent a test email from my fasthosts account to my plusnet account, using the relay.plus.net as the outgoing mail server in mac mail...well, the email left, but it didn't get delivered yet. Does this indicate anything?

[#5]

14 hours later....has it arrived yet
If not I would start checking spam folders

[#6]

If you were using Fasthosts SMTP, were you authenticating to it?  Do you have it set to use a username and password when sending an email?