I have found this to be internal IDS Traffic alerts. Seems like an IDS false positive. Our IDS settings are set to prevention and security, curious if you're using the same?
We never got a satisfactory answer from Meraki... they basically passed the buck. We are still seeing some "allowed" traffic flagged under the Exim event, almost exclusively from Google and Amazon addresses. Continuing to review with another one of our security vendors.
I'm also suddenly receiving a bunch of these IDS alerts. Ours seemed to start after we added a second location/Meraki MX and configured a site-to-site VPN tunnel. The alerts are triggering on traffic traversing the site-to-site VPN tunnel, both on traffic destined to an internal server and outbound to the internet (AWS & Akamai addresses, etc).