FortiAnalyzer "Number of emails" how is this measured?

In the FortiAnalyzer reports section 'Emails' can be selected as content.
Does anyone know (or can point me to) how these numbers are calculated?
'Top Recipients by Number of Emails' for example gives me a public IP address and a count, [ "1,734" ] for example.

How do Fortinets 'know' - counting traffic on port 25?

What happens if I have an application using this port and my email is on 587?

Any replies gratefully received.

P.