Outlook 365 (classic) freezes syncing subscribed folders via IMAP

Hi Jan,

Your post is reminding me of an IMAP account with a POP import not completing. 

IMAP with Google setup in Outlook Classic has a few twists to it when migrating from POP to IMAP. POP/PST can have a lot of email with many large attachments and if you imported into your Gmail IMAP account via Outlook Classic, you might not have allowed Outlook to finish the uploading process, this is very common based on my past client calls. If your Gmail was also added as IMAP on another device, your IMAP/Gmail account (and esp if an older Gmail account) can have a lot of data to sync. One trick is to close Outlook Classic, then turn off 2 folders in the Gmail labels: All Mail and Important. Those will not sync to your Outlook profile. I mention all this as you mentioned the move from POP to IMAP. 

Then let it rest, keep your computer on for a couple of days, leave Outlook open and then see if it stops synching. Due to internet speed and any other issues such as VPN, AV or a firewall, the time might be delayed.  

If you did all of this and are still seeing snags in the synching, you might have a very large email with an incredibly large attachment stuck. I have found this more times to be the root cause. Search in your "search folders" for large attachments, find the enormous size emails and then I would manually move them off the profile to a PST vs deleting, makes it final and can be stored in a PST for viewing now or not get caught in the sync again with the trash. 

As mentioned, VPN, AV and firewalls can cause your issue as well. 

OneDrive should not be a problem here as IMAP OST is stored locally by default. 

I hope this advise helps,

Lisa Hendrickson, Microsoft MVP (Outlook)