Implement the HELO command.
Allow to record multiple RCPT statements.

They’re perfectly legal to be repeated, and if they are we need to
record them all.
Handle EHLO and HELO correctly as well as empty params.

Complain if a HELO is issued without any parameters.
Implementations for MAIL From and RCPT To.

So far we’re just recording and checking that things were performed in
the right order.