Outgoing sales invoice emails very slow or not working

I've had this same ongoing issue to the point where a new computer was ordered, and it still happened. I finally worked out a solution that seems to work every time. It's an easy fix, even if I have to do it every few months.

Go to Settings → Apps → Installed Apps → Microsoft Outlook (I have the screenshot below installed).

Click on the three dots and select Modify → Yes.

Select whether you want to do a Quick Repair or Online Repair.

I've always selected Quick Repair, and this has fixed the issue every time.