The first thing you should do is to check that you have an MTA on Pandora FMS’ server, that the port you use for SMTP is open and the MTA service started.
The next thing is to check the properties of your pandora_server.conf and make sure that your mta_* parameters are correct and your MTA Mail Server can send your messages. In case there is no configuration in the server mta_* parameters, the mail section in the Pandora FMS console should be correctly configured in the Setup -> General Setup section.
If you are using MTA authentication make sure that Sendmail.pm (usually in /usr/share/perl5/Mail/Sendmail.pm) is version 0.79_16 or lower.
Version 0.79 does not support AUTH methods. If your version is old, just replace it with 0.79_16 . Another option is to install from CPAN (http://cpansearch.perl.org/src/MIVKOVIC/Mail-Sendmail-0.79_16/Sendmail.pm).
Older versions work perfectly for MTA without authentication.
In case you are using Pandora FMS Enterprise server, you should omit this last check because the server binary includes the updated library.