Envelope recipient support not determined by TO: header fields in API (SMTP Parity)
In SMTP you can set envelope recipients that are not listed in the To: header fields (think BCCs here).
We have a need for this to send unique emails to individuals, while having the To: header in their email client appear as if it was to a group that this user is a part of.
Currently, this is not possible via the Mailgun API, but only via SMTP. Since Mailgun recommends using the API, I would love to see this feature included so we can make the leap over to the API instead of using SMTP for these particular emails.