Certain Mail services are rate limiting emails being sent to them meaning they only accept a few emails an hour from our mail servers. We're in discussion with them to get a white-list in place to bypass this, but there's no ETA as to when this might occur, as the block is in place at their end.
Mail services currently affected are:
Our current recommendations are to change the Outgoing Mailserver to one of the below, and to add an SPF record. Please open a ticket
for more assistance on this matter.