Some emails are not synced to D365
Dynamics 365 and Exchange integration is plug & play with some minor configuration...I just don't think so.
Lately, I had an issue that emails were not syncing to the Dynamics 365 using Exchange server via server-side synchronization. We spent like a month of intense monitoring and changing every single parameter in hope that emails will finally get synced without any issue.
Here are some of the tips that we used on our journey which finally made our integration work flawlessly.
Update to the latest Dynamics 365 version
The most basic thing to do is to update to the latest release of Dynamics 365, but of course, I know that it's not always the best option and most people out there are considering it as the last option if everything else fails.
Just trust me that more than few times I regret that I didn't do an update first before I used too many hours of researching the issue.
Microsoft is adding tons of updated/hotfixes with every release that are not mentioned in the release notes. Last few versions had quite some updates related to the server-side sync mentioned in the notes.
The same email address on multiple entities
You should not use one email address on multiple OOB entities. It doesn't matter if you are using it on 2 same records like contact entities for example or you just use it on 2 different records like on one user entity and one contact entity.
You need to ensure that you just have one record on the instance that has a unique email address that is populated in the OOB email field (eg. emailaddress1,emailaddress2,..).
Syncing same email address on 2 instances
You can forget to disable email sync on the development instance when you finally move your email integration to the production instance.
The first step is to remove server profile from the mailbox on the development instance. Go to the mailbox that you want to sync on production, clear the server profile field and hit save button.
The next step you need to do it test & enable the same mailbox on the production instance, but you need to be sure that you select the “Sync items with Exchange from this CRM org only” checkbox on this step.
After those steps, you are sure that emails will be synced only to the production instance.
Change mailbox ownership
The most strange tip is to change ownership of the mailbox form the TEAM to the USER.
I know that this sounds ridiculous, but it actually helps in some situations. We came to this solution after talking to MS support that after a while suggested doing so and after that everything just started working like a charm.
I just hope that some of this tips will save you some time while investigating the syncing issue like this one.