I set up Postfix to use Sendgrid to relay Voicemail emails (thanks @billsimon)
My issue is that in the email body there is a link
“Visit http://AMPWEBADDRESS/ucp to check your voicemail with a web browser.” which is being sent as “Visit https://u25628.ct.sendgrid.net/ls/click?upn=etcetc_long_string_edited”
I had, prior to configuring Postfix, successfully changed that link to the IP of the box Settings>Voicemail Admin>Email Config but since configuring Postfix it ignores that and sends that URL instead.
Then I tried putting back to AMPWEBADDRESS after setting FreePBX Web Address in Advanced Settings to the IP address of the box, but that had no effect.
emailbody=${VM_NAME},\n\nThere is a new voicemail in mailbox ${VM_MAILBOX}:\n\n\tFrom:\t${VM_CALLERID}\n\tLength:\t${VM_DUR} seconds\n\tDate:\t${VM_DATE}\n\nDial *98 to access your voicemail by phone.\nVisit http://192.168.16.22:81/ucp to check your voicemail with a web browser.\n