Fallback to sending emails with the host
Travis Pflanz
Currently, MailPoet has a whole list of things it cannot do - https://kb.mailpoet.com/article/292-choose-how-to-send-your-wordpress-websites-emails
Via email, MailPoet told me that if an attachment, BCC, CC or multiple TO email addresses are included, then MailPoet does not fallback to sending via the website's default method (WordPress uses the web host by default)
Philipp Stracker
Also fallback to send via host/SMTP when MailPoet sending is disabled for some reason. For example, the subscription limit is reached, MailPoet does not notify me but suspends sending. When too many hard bounces happen, MailPoet also suspended my sending ability.
To avoid problems in those cases, I would be very happy if MailPoet would simply "downgrade" to sending via my host or a custom SMTP connection instead of not sending at all.