I got the certificate error when I first configured the mailbox. I did something wrong - are you sure in username you are entering "email@example.com" instead of just "geekzone"? Because that's what caught me here.
Once I had the full username configured the client had no problems. I tried both manual and automatic configuration procedures - obviously automatic failed but then asked for server details and it worked from there.
You can also can try entering the mail server address as mailserver:110 for pop and mailserver:25 for smtp to force those settings/protocol. See if it helps.