Mail Transport Agent (MTA) are software that receive and deliver email to other MTAs or MDAs based on the destination address. The users generally uses Mail User Agent (MUA), while MTA works in the background.
Example of MTAs:
For a comparison see:
Boot process gets delayed / stuck at MTA startup
Depending on your specific network configuration it is possible that the mta service (actually called "exim4" under wheezy), starting at runlevel 2, waits for a network connection to be established. But in runlevel 2 there is no networking active. This can result in a very long delay at boot time. In order to resolve this issue one can disable the start of the exim4 service for this particular runlevel. For each runlevel <n> there exists a directory /etc/rc<n>.d/ containing symbolic links to the services that are started in this runlevel. To disable the exim4 service for runlevel 2 one has to cd into /etc/rc2.d/, rename the symbolic link so that its name starts with the upper case character K and issue a update-rc.d exim4 defaults command as described in /etc/rc2.d/README:
The scripts in this directory are executed each time the system enters this runlevel. The scripts are all symbolic links whose targets are located in /etc/init.d/ . To disable a service in this runlevel, rename its script in this directory so that the new name begins with a 'K' and a two-digit number, and run 'update-rc.d script defaults' to reorder the scripts according to dependencies. A warning about the current runlevels being enabled not matching the LSB header in the init.d script will be printed. To re-enable the service, rename the script back to its original name beginning with 'S' and run update-rc.d again. For a more information see /etc/init.d/README.