Not All Emails Sending, Intermittent Failures

I have been using Mailgun for over a year on a dozen or so sites, all on the same dedicated server that we have. It’s been working flawlessly until about a week ago where not every Gravity Form notification is being sent out. I’ve confirmed this via the Gravity Form logs on 3 of our sites. I’d guess that 20-30% of them are failing, but there are no patterns or things that stick out. For any particular entry, one notification will succeed such as the User Notification, where the other E.g. Admin Notification will fail. One day all will work, one day a few may fail.

2017-09-15 16:31:21.215797 - DEBUG --> GFCommon::send_notification(): Starting to process notification (#537f9a47dbfcb - Admin Notification). 
2017-09-15 16:31:21.231738 - DEBUG --> GFCommon::send_email(): Sending email via wp_mail(). 
2017-09-15 16:31:21.231791 - DEBUG --> Array
 {contents of email have been removed}
2017-09-15 16:31:26.756771 - DEBUG --> GFCommon::send_email(): Result from wp_mail():  
2017-09-15 16:31:26.756966 - ERROR --> GFCommon::send_email(): The mail message was passed off to WordPress for processing, but WordPress was unable to send the message. 
2017-09-15 16:31:26.761140 - DEBUG --> GFCommon::send_notification(): Starting to process notification (#5390cce5317c3 - User Notification). 
2017-09-15 16:31:26.768512 - DEBUG --> GFCommon::send_email(): Sending email via wp_mail(). 
2017-09-15 16:31:26.768692 - DEBUG --> Array
 {contents of email have been removed}
2017-09-15 16:31:27.229549 - DEBUG --> GFCommon::send_email(): Result from wp_mail(): 1 
2017-09-15 16:31:27.229613 - DEBUG --> GFCommon::send_email(): Mail was passed from WordPress to the mail server.

That shows that the User Notification was good, but then ~1.5 seconds later, the Admin notification failed.

I reached out to Gravity Forms and they claim that the issue is downstream from them as they are successfully trying to pass the mail every time.

Others have said there could be some network issues between our dedicated server and Mailgun’s server(s) – I have opened a ticked with my host.

Could it be internal, such as WordFence or similar?