If you are running Win 10 and use Office, you may find that email from outlook is stuck in your out box. I'm on Office 2013 but it seems that Office/Outlook for many versions may be affected.
 
It's a bug and the net is full of fixes. It seems that the Win 10 install corrupts the outlook database in some way. There's a fix, just Google Win 10 outlook send problems. I was on the phone with tech support at my ISP and they had no idea why I couldn't send email. As usual Google had the answer right away.

I dumped Outlook, now use thunderbird and never looked back. I had enough of the bloat.
I appreciate some have to use it because sysadmins force them to...
 
Cheers...

Look out for Outlook.
 
I still use it because I have WAY too many emails with registration codes, for example, and have found no good way of migrating my 70MB+ Outlook database to another platform.
 
That said, I've had no trouble with my Outlook install and Win10. I have the click-to-run version of Office 2013. I don't use Outlook.com, nor do I have to connect to a corporate Exchange server.

Check:
https://support.mozilla.o.../switching-thunderbird

Doktor Avalanche
Check:
https://support.mozilla.o.../switching-thunderbird

I'll give this a try. There is a method for Gmail, but it doesn't work with click-to-run versions, and, in any case, is slower than molasses. I really hated the way MS dumbed down Outlook 2103 w.r.t. calendar and tasks.

Ya, once I installed 2013 I contacted MS to see if I could get an older version. I can still hear the laughter. I'll check out Thunderbird.

I haven't seen this issue, knock on wood. I'll give up Outlook when they pry it from my cold, dead hands. I have a pile of routing and formatting rules that I never want to try to replicate elsewhere. Besides, I adore Outlook. The Office version, not the brain dead mail app with win 10. To each his own.

Been using Outlook 2016 and not had this issue at all. Everything is running swimmingly.

Btw I assume you have compacted all your mailboxes, and made sure your firewall isn't getting in the way...

Also make sure your antivirus is not pointing to your mail storage folders on your hard drive. Any virus scanning should be happening at port level. If it's done on the filing system you will experience the symptoms you are talking about. Learned that one the hard way with MS Exchange servers.

Cheers..

Running smoothly here.

You can convert all your emails to PDFs using the Adobe reader addin. That gets you away from having to keep outlook just for old emails.