The staple cms:send_mail tag used by Couch to send emails internally uses PHP mail() function which is sometimes not available on certain hosts. As an alternative, we can use the phpMailer library to get the mails through.

Following thread explains how -
viewtopic.php?p=16802#p16802

UPDATE!
A 'phpMailer' addon is now available to make the process easier -
viewtopic.php?p=22911#p22911