Ideally, GP would have an option to setup a "From" account along with the "Reply To" account for each Message ID, and let GP pass the network credentials to Exchange so each user doesn't have to login separately.

An AP department will usually have a common shared email account to send remittances and receive vendor emails. If GP would allow a "From" account to be setup with a Message ID, then AP department employees wouldn't have to use their own email addresses, and all sent emails would be available for audit/review/resending. Without this feature there is no method to ensure all emails sent from GP are available from a single account.

Logging into Exchange is required when setting up email addresses and/or generating vendor remittance emails in GP. Windows, AD, Exchange, and GP are all Microsoft products that pass network credentials... having each user login to Exchange should not be necessary.