How to list how many letters from the site have already been sent?

Question

PHP

There is a form which sends letters.
It is necessary to add in the letter the number that goes on the account this sent letter from the site. Please tell me what function or where to dig to implement this?
That is, the point is to see in the mail what number this letter is coming, like (1, 2, 3, etc.)

Answer 1

The number must be stored on the server, it can be in a file, it can be in the database. The easiest way would be to implement through the database.

Answer 2

Look at the sending engine, implement a counter there, what's the problem?