Email in arrivo non ricevuto

[JammyDodger]

@Justinvolved

I think in the troubleshooting section at point 2
Did the e-mail even make it to mail-receiver ?
It would be nice to add a line to check if there is a firewall that blocks incoming smtp traffic, ideally maybe even link to something describing the exact ports that should be open.

I have added a line to the wiki guide.