Sometime in the next 24 hours we'll be changing over the SSL certificate for mail.messagingengine.com to a new one. In theory, users shouldn't notice any change at all. However there are two changes in the new certificate that might affect some users.
We've checked that compatibility with this new certificate should be good, but as always, there are edge cases and some users may have issues. If you do have any problems, please email me directly at firstname.lastname@example.org with details of the device you're using and the error you're getting.
The reason we're changing is that Digicert offer more flexibility with their SSL certificates, such as wildcard certificates with multiple "server alternative name" options.
Update: I rolled out the new certificate, and shortly afterwards had some reports of problems with Eudora/iPhone/Thunderbird/etc. I contacted Digicert support who were very helpful. Turns out I'd forgotten to RTFM fully, and hadn't included the chained certificate in the PEM file. I've now done that, so things should be better for Eudora/iPhone/Thunderbird/etc users that were having problems.
Update: Unfortunately for Eudora users, it seems Eudora does not come with the required root certificate built in. This means Eudora users will still see an error message with the new certificate. Fortunately, it's easy to fix this, just follow the directions here to add the list of trusted certificates in Eudora.