Actually, the mailserver is working fine, the email:

1) is in your Junk folder, which you never review for false positives

2) is in a subfolder, due to a rule you forgot about or wrote poorly

4) is moved/deleted/junked due to an errant keystroke

5) never arrived at our server due to the sending server's horrid misconfiguration

5) was quarantined by our server due to the sending domain's horrid misconfiguration

6) is in your main inbox – the problem is that you need a break

#JustITThings