Android forums email stuck in outbox 2016

I had people complaining that their e-mails weren't sending until they restart their Outlook. The only time I had this complaint was when I switched over to Bearer Authentication in Exchange 2016 as I wrote about in this reddit post: If you are having issues with Outlook 2016 and on prem Exchange, then it probably isn't ActiveSync. Anyone have an idea based on what I've presented? I am more than happy to provide more information as necessary. I'm at wits end as I'm getting pressured by multiple managers and the end users as well. We have migrated a few test users and none of them have reported this problem.

I am just waiting for management to finally give me the go-ahead to migrate everyone, but for now I must wait (impatiently). We are currently running Exchange 2016 hybrid with O365. The people complaining are blaming this on our ongoing O365 migration. I've confirmed this issue (on phones) happens on both WiFi and cellular (we are in an excellent coverage area). All users that have reported the issue have healthy mailboxes with AS and OWA enabled. I've poured over ActiveSync logs and have found nothing out of sort.

I believe Exchange, our file servers, and the network infrastructure are the only things they do not have access to). They also have their own IT team (no Exchange access, but their own GPOs, file servers, storage, etc. Note that this issue is only being reported by the Police Department (I work for local government). This has been confirmed to happen with the Mail app on iPhones, the Gmail, Outlook, and Samsung email apps on Android, and Outlook 2016 on the desktop. Over the past couple days, I've received a few complaints from users saying they will send an email out and it just sit in the outbox.

Android forums email stuck in outbox 2016