Voicemail Pro is usually easier for me on installs,
looks like your not going to use the VM Pro if you switch to embedded. Unless there is a trick to embedded sling-shoting to the VM PRO server. ( I never claim to know it all just what I've done and messed up before)
The trick to VM Pro is to have a SMTP outbound server that will allow relay. From experience Gmail does not want to allow mail out, or its a fight to get it to allow it out. problem could be on the business platform of Gmail, I never tried a personal account.
The most reliable test for me is authsmtp.com for outbound service, I am guessing if everything is definitely set correctly in your VM Pro client under administration=> preferences => general => email=> smtp sender, as holdmusic34 said it may not be Avaya but how mail is allowed/ handled by the mail server( either in-house or hosted)
Authsmtp is a paid service for outbound mail ( pretty cheap for a whole year) I use it to as a solid server sending out when troubleshooting other email server configurations that are fighting back. Lets me know if it is me because it has a console to see mail sent out and an error log if you do have something wrong as in ports or credentials or even address mistypes.
This is just a direction to head into if you want to dig into it deeper and see more. ( especially if you don't understand the monitor in IP Office. Its kind of like drinking from a fire hose if your not familiar with it.)
Jerrberr