This is a little out of place, but since this is the best forum I’ve been to regarding Linux/Asterisk/FreePBX I figured I’d ask here.
It’s a newbie type question so there’s your warning! 
Whenever someone messes with the hostname of our FreePBX server we stop getting voicemail to email attachments. We use a mail server to send the actual emails to the recipients. Under the mail logs on the FreePBX I see the message that the user is unknown. My question is…if I wanted to change the hostname where do I go to make sure that the user unknown message never comes up? Do I do this on the mail server itself, or on the Asterisk/FreePBX box that actually generates the email?
I was thinking that sendmail on the mail server looks up the hostname and compares it to what it has on file. If there is no match then it refuses to send the email, and puts up the user unknown message. Is this the case though?