Update: Upon more detailed testing, occasionally waiting for an extended period of time leads to it unfreezing and resuming as normal.

However, the duration of the freeze still makes it problematic for regular usage.

Are you using Avast or McAfee>

From my original post "- I have no antivirus/antimalware software running".

Upon further, further troubleshooting, it appears that waiting various extended periods of time leads to unfreezing. However, it's unusable to have freezes for several minutes every few sent emails.

It may be related to the copy to sent folder feature which was a reported problem in older versions. Perhaps a change in 38.x made this issue return.

Will report back as I test more.

Disabling the "copy to sent folder" feature does not resolve the issue, so that isn't it.

Will report back if I discover anything else.

And since when has someone saying they have no anti virus running meant anything. I get that all the time. If I take notice waste a load of time and find out that they have an "internet security suite" or "Windows Defender" so I ask specifically about program names. It appears to work.

I suggest you try the standard diagnostics instead of wandering to other threads with your less than helpful comments. I choose to ignore information because usually those supplying it do not have the technical skills to be aware the information they supply is rubbish. If you want your statements given more weight, demonstrate your technical skill in your comments. Is your account IMAP or POP. What mail provider what internet provider. All those little things the technically skilled are aware is useful information.

To diagnose problems with Thunderbird, try one of the following:

• Restart Thunderbird with add-ons disabled (Thunderbird Safe Mode). On the Help menu, click on "Restart with Add-ons Disabled". If Thunderbird works like normal, there is an Add-on or Theme interfering with normal operations. You will need to re-enable add-ons one at a time until you locate the offender.
• Restart the operating system in safe mode with Networking. This loads only the very basics needed to start your computer while enabling an Internet connection. Click on your operating system for instructions on how to start in safe mode: Windows 8, Windows 7, Windows Vista, Windows XP, OSX

If safe mode for the operating system fixes the issue, there's other software in your computer that's causing problems. Possibilities include but not limited to: AV scanning, virus/malware, background downloads such as program updates.

I see. So it seems what I've heard about SuMo is true. You can close this ticket. I won't waste my time here anymore. Shame. SuMo had a lot of potential.