Hi @grepnerd
your configuration looks a little bit curious.
If you use that mail address, notjustcamden.uk is relevant, the part after @.
D:\temp>nslookup -type=MX notjustcamden.uk.
notjustcamden.uk MX preference = 10, mail exchanger = a.mx.notjustcamden.uk
So a client would connect a.mx.notjustcamden.uk, so you should have a certificate with that domain name.
And you should use that domain name as pop3 server / imap server too.
But the A of notjustcamden.uk is 46.43.2.88, a.mx.notjustcamden.uk has the same ip address.
So you don't really need a.mx.notjustcamden.uk, you can use notjustcamden.uk directly. There you have already a certificate, you can re-use it.
But that requires that you change your MX record.