These are the api endpoints to allow connection with this plugin.

Send campaign immediately to specific lists or segments. (Ability to select more than one segment at a time is a bonus) (compulsory)
Accept custom html as content (compulsory)
Verify email addresses programmatically (important)
Send test email to any email address (important)
Save campaign as draft in the ESP without sending it (important)
Mergetags for email personalisation (important)
Ability to set preview text (bonus)