Outlook 365 Distribution Groups no longer showing new email.

[SChicora]

I have four users in my organization that are experiencing distribution groups displaying x amount of new emails counter, but no new emails are visible. Each user is experiencing this with a completely different distribution group.

1. Turned off cache mode and turned it back on. (This brought in the newest group emails, but it once again stopped displaying newer ones)
2. Renamed the .ost file and let it rebuild another. (This brought in the newest group emails, but it once again stopped displaying newer ones)
3. Made a new Outlook user profile. (This brought in the newest group emails, but it once again stopped displaying newer ones)
4. Using Outlook on the web does show new group emails.
5. Un-joined and rejoined users to their respective distribution groups. (This brought in the newest group emails, but it once again stopped displaying newer ones)

[#2]

How big is the local profile? Most likely you will have to reduce the amount of emails being cached locally.

[#3]

These particular users have quite large profiles. I will reduce the amount of email cache and get back to you on results.

[#4]

I checked their OST size and cache allocation and found nothing of too much size. The limit is to keep a cache no more than a year. OST files are less 30GB in size.

[#5]

30GB is still quite large, especially if the machine is older (or uses HDD).

[#6]

I am having a similar issue. 2 weeks ago I could no longer see the incoming emails to my Microsoft Groups. I can access them on OWA but not through the desktop app. Administrator indicated that nothing had changed on his end. He tried deleting and re-adding me to the group, but still have the same problem.

[#7]

I believe the default maxOST size is 50 GB for the latest versions of Outlook. This shouldn't cause any users to stop receiving emails t 30 GB though it might affect the application performance drastically causing it to freeze/crash frequently.

[#8]

We spoke to a Microsoft Tech and he guided us to an Admin setting that needed turned on. Group emails for all employees are coming in properly now.
Admin Center > Settings > Org Settings > Modern authentication > Check (Turn on modern authentication for Outlook 2013 for Windows and later (recommended))

[#9]

This worked perfectly