I've had this issue for several years now. Happens maybe once for every 10 emails I send. I use Firefox with LastPass. I've had it happen on multiple different computers over the years and many different versions of SmarterMail.
Today, when it happened, I brought up developer tools and I noticed there was no activity in the Network tab or Error Console when I hit Send. I was very surprised by that. I expected an attempted POST but there was none. But then I noticed there was no network activity when I type in different names in the TO field and I, as a web developer, am confused by that. How is it loading the suggestions without pulling from the server?
Time zone and time on my server and laptop are the same.
To me, time doesn't seem to matter. I've noticed the Saving Draft will get stuck on the first attempt and then it is blocked for all time from then on.
I just tried to replicate it 8 times with no luck but I did see there was consistently an error in the Error Console for all those messages where it did work fine:
Content Security Policy: The page’s settings blocked the loading of a resource at eval (“script-src”). angular-v-100.0.8552.30682.8db62ec08a6a500.js:1295:6
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