AWS SES delay, but only with Mimecast

I've got an interesting one here, and I don't know where else to go with this at this point. We started using SES to deliver emails from our application with 2 dedicated IP addresses. We have a few clients who use Mimecast to filter emails, and they have been reporting delays. At first, I had assumed the delays were due to being grey listed by Mimecast. The client confirmed they had added us to their grey list bypass filter for Mimecast. I then had the mutual client reach out to Mimecast for suspected issues at their servers. Mimecast advised the client that the total transit time through their system was about 2 seconds, and the issue wasn’t their systems.

When I review the logs I’m seeing a great deal of delay in ‘processingTimeMillis’ for ‘Delivery’ I’m consistently seeing this with Mimecast destinations. Thus far the system has 4496 delivery emails in the logs. Below is a breakdown of the averages. Total:

Going from a handful of seconds to a few minutes is quite a jump. When reading on what ‘processingTimeMillis’ I see it says

The time in milliseconds between when Amazon SES accepted the request from the sender to passing the message to the recipient's mail server. I take this to mean the time it takes SES email server to deliver the message to the other server. My assumption is this time includes retry attempts, disconnects, and waiting for network connections.

Is there anything I can do to obtain more information on exactly what is causing this delay? I’m sure once I have the exact cause of the delay, I can approach the appropriate party for resolution. How can I obtain the records to show what is causing this situation?

I appreciate any guidance you can provide!

Methodology of how I arrived at some of the statements above:

To help get more information I've enabled logging and I've downloaded the logs to my computer. I eventually turned off click tracking as well as open tracking as that was causing some unrelated issues and another post indicated that could cause delay as well. I've not noticed any change to this issue. From reading https://aws.amazon.com/blogs/messaging-and-targeting/three-places-where-your-email-could-get-delayed-when-sending-through-ses/ I see 3 basic causes. I think we can safely rule out an issue before or after SES as we are just looking at SES delivery times here, this rules out the first and third point. The second one indicates it could be the ISP, but they claim everything is good.

How I obtained the stats: I ran all these commands from the log folder date, etc: 'logs/2022/07/18' From there is an hourly folder, then the individual log files. Average processing time for delivery: grep -h delivery / | jq '.delivery | "(.processingTimeMillis) (.smtpResponse)" '| tr -d '"' | awk '{ sum += $1; n++ } END { if (n > 0) print sum / n; }' Average processing time for Mimecast: grep -h delivery / | grep mimecast| jq '.delivery | "(.processingTimeMillis) (.smtpResponse)" '| tr -d '"' | awk '{ sum += $1; n++ } END { if (n > 0) print sum / n; }' Average processing time excluding Mimecast: grep -h delivery / | grep -v mimecast| jq '.delivery | "(.processingTimeMillis) (.smtpResponse)" '| tr -d '"' | awk '{ sum += $1; n++ } END { if (n > 0) print sum / n; }' Command used to determine how many in emails in a day: grep -h delivery / | jq '.delivery | "(.processingTimeMillis) (.smtpResponse)" '| tr -d '"' |wc -l