SmarterMail Enterprise
                       Build 7793 (May 3, 2021)
                   

This has come up on MailOps...  I noticed since the May 10th updates for Windows, MAPI is now broken on Outlook 2016.  For Exchange it seems like you can downgrade Outlook:

I tried on SmarterTools and doesn't seem to work.  From the SmarterMail logs everything seems okay.

[2021.05.11] 23:54:28.535 [{50A93887-2588-4C56-BC89-8FA0B0B81D78}:43] Started Method: [RopOpenFolder] Caller: [eric@truenet.com] Owner: [eric@truenet.com]
[2021.05.11] 23:54:28.535 [{50A93887-2588-4C56-BC89-8FA0B0B81D78}:43] Finished Method: [RopOpenFolder] Caller: [eric@truenet.com] Owner: [eric@truenet.com]
[2021.05.11] 23:54:28.550 [{50A93887-2588-4C56-BC89-8FA0B0B81D78}:44] Started Method: [RopFastTransferSourceGetBuffer] Caller: [eric@truenet.com] Owner: [eric@truenet.com]
[2021.05.11] 23:54:28.550 [{50A93887-2588-4C56-BC89-8FA0B0B81D78}:44] Caller: [eric@truenet.com] Owner: [eric@truenet.com] Folder Id: [70038] Sync Type: Contents Sync Flag: Fai Sync Flag: Normal Sync Flag: Downloading Buffer Size: 47806 Maximum Buffer Size: 31680
[2021.05.11] 23:54:28.550 [{50A93887-2588-4C56-BC89-8FA0B0B81D78}:44] Finished Method: [RopFastTransferSourceGetBuffer] Caller: [eric@truenet.com] Owner: [eric@truenet.com]

Outlook says it can't log in: Cannot expand the folder. The set of folders cannot be opened. The attempt to log on to the Microsoft Exchange has failed.  

*Edit: not sure, but it seems to be working this morning.  Definitely a strange issue, so may be related.