What is a SRV record.

Service record.
Provides information on available services.
A SRV record has four fields and a unique system for naming.
The naming system is an underscore followed by the name of the service,
followed by a period, and underscore, and then the protocol, another dot,
and then the name of the domain.
four fields:

1. Priority, just a in MX records
2. Weight, Hits will be assigned proportionately by weight.
3. Port, the port of the service offered
4. Hostname

example:
_smtp._tcp.example.com. SRV 10 5 25. mx.example.com