To Prevent Mail Client Keep Asking To Verify SSL Self Signed Cert

Hi, to prevent mail client (Outlook, Opera Mail etc) keep aksing to verify self signed SSL Cert should I use proprietary SSL certificate ?

Is Comodo Positive SSL (cheapest SSL) is enough to prevent mail client keep asking to verify SSL ?
How about FREE StartSSL cert ? Is it also could prevent mail client keep asking to verify SSL ?

I know this is not related to ISPconfig matters but I cannot find answers from anywhere. Thank you.

 

You can use certs from startssl. The verify appears for self-signed-certs only.