Look into replacing Mailgun

[Riscky]

Mailgun is a fine mail API provider, but we have one big problem with it: it does not always deliver email.
This is most notable with Outlook accounts, that rarely ever get mail through in one go.
MailChimp, which we use for email lists, always deliver, but doesn't have an API for sending batch emails (just for updating groups).

An alternative could be Sendgrid, which seems to have great delivery.
It might be pricier then the free Mailgun instance we currently have though.