At Porkepic, we moved our biggest customer to the online SaaS version of C3 last week. New UI, new color, same fonctionalities. Most of the transfert was done without any harm thanks to my partner Jean-Francois. 

We have a feature that allows a user to forward his emails from his mailbox to the software where he can link it to contacts, clients, contracts, projects, etc. We rely on Sendgrid parse API to do so. Basically, it receives the emails for you, parses it, and sends it to your web application as a POST. Pretty simple, yet kind of powerful.

As soon as Monday morning 8h, we were receiving complaints that the forwarded emails did not reach the application. Some of them did go through, but not all of them and in the software world we like it when it is true or false, but not sometimes. It seems some were dropped along the way but the sender was not receiving delivery failure emails. 

We contacted Sendgrid support, which is quick and as helpful as they can be, and they had no clues of what might be happening. To make a long story short, to me it seems that their mx.sendgrid.net server was not responding to all queries from SMTP servers trying to send mail to it. So the SMTP server queued the email for later delivery (as the mx server was reachable) which can happen within 5 min or 5 days. So we modified our DNS records to point to mx1.sendgrid.net, mx2.sendgrid.net and mx3.sendgrid.net and it did the trick!

So if you are experiencing the same problem with the parse API from Sendgrid that might just be your solution!