We're updating SSL certificates for our email servers today. The old serial number is 12AD7B. The new serial number is 130CAB. Users who don't have the CAcert root certificate installed may see errors. This does not affect webmail or outgoing SMTP. Details on http://aa.net.uk/cacert.html
We have a new email proxy that should fix these problems; those affected by this can try setting their incoming mail server to mail.aa.net.uk (TLS only, no STARTTLS). Please note that this is not yet "launched" and is therefore not yet officially supported. More info here: https://aastatus.net/2407