What email client do you use?

And what server details did you configure the accounts with in your client? / Are they configured with if you used the auto-configure utility? (I would guess you used the auto-configure and they are set to use mailX.gridhost.co.uk) - where X is a number.

I'm not trying to start a rush on tinfoil hats, this is just how SSL works on shared hosting.

GeeTee - fascinating stuff. Thank you for such a comprehensive and helpful response. I shall look carefully at TSOHost.

Thunderbird. The auto-config can be a nuisance, but it is possible to use it then make strategic alterations if necessary before saving the configuration.

Depending which of their hosting servers you are on, it can default to non-SSL. Unless they've now corrected that server. The non-SSL system allows mail.mydomain, the SSL ones uses their full server name, of the form username@jupiter.servers.rbl-mer.misp.co.uk.