Recently had a customer that was getting 15 – 20 minute delays in receiving emails from MailChimp. The same email to services such as Office 365 , Gmail and other email providers came through straight away.

We had whitelisted Mailchimps IP’s per below from Greylisting and Spam to no avail. The Message tracking proved that the message wasn’t hitting the Administration console for the 15 Minutes delay

Source IP Ranges (n.n.n.n/x): 
205.201.128.0/20
198.2.128.0/18
148.105.0.0/16

We lodged a Mimecast support ticket and got this escalated to Level 2 and waited a week for a response. In the end a retry of the issue with the technician came through straight away which means there must have been a block somewhere in Mimecast