This is a list of known issues with email and the possible solutions:
Certain hosting providers do not relay emails using off-domain email addresses
Normally, emails go out using your WordPress admin email address in the FROM header. Some servers, however, will not process outgoing emails that have an email address from gmail, yahoo, hotmail and other free email services in the FROM field. Some servers will also not process emails that have an email address that is different from the email address associated with your hosting account in the FROM field.
If you are with such a webhost you need to make sure your WordPress admin email address is tied to your hosting account or use the plugin settings to override the FROM email address (Classifieds > Settings > Email):
- Update the FROM email address for outgoing emails setting to use an email address that matches the domain associated with your hosting account.
- Uncheck the Use sender email for reply messages setting.
Some email servers have problems handling email addresses that include the name of the recipient
If emails sent by the plugin are not being delivered properly, try unchecking the Include the name of the recipient in the email address setting so that email messages are sent to firstname.lastname@example.org instead of John Doe <email@example.com>.