Assuming you have TB 115 on this setup (latest version for W8.1), what are the gmail settings, and are you using a VPN? Settings should be imap.gmail.com, 993, SSL/TLS, OAuth2, email address; smtp.gmail.com, 465, SSL/TLS, OAuth2, email address. smtp.google.com is not a valid server.

Actually, trying version 99.0b2 (32 bit) right now but had 115 installed before and had similar results. However, when I tried this earlier version, I was able to send 1 email before it stopped sending. On 115 it wouldn't even send one. I apologize for not sending server settings. I've attached them. Since whatever past version of TB, I've just used the auto setup as it's always connected to Google servers with no problems.

Not using a VPN, only Windows Defender on client, and Windows security on host. I tried both "default and external virtual network switches" in Hyper-V. Both allow me to use the host machine's network adapter. So, both settings have full internet access and can receive emails flawlessly. Tried turning off firewalls in both Host and Client.

Interestingly I was able to send 1 test email again after starting the VM session today before it stopped sending again. However, that test took almost 10 minutes to hit my inbox on my other account. I almost want to think it's a Google-side permission that is not allowing me to send. But I allowed TB to connect with Google when I was installing.

I've logged in to the admin console for my Google Workspace account and can't find anything. But I'm no expert when it comes to workspace.

I've no experience with VMs, but I'd try creating a new profile with 115.16.2, to ensure the latest OAuth2 settings, add a personal gmail account, and see if it works. If it does work, add the workspace account and test. If it doesn't work, I think the issue is with gmail and VMs.

The reason for testing in the new profile is to avoid any issues with re-using old profiles that might have been used with various versions.

I wanted to follow up on this and let you know that it's fixed!

Now, it could be one of two things I did over the weekend.

The first is that I was suspecting hardware issue in my computer as this was the only one that this was happening to and because of some BSDs I had experienced last week. So Google said I should run MS memory diagnostics. It showed that the memory was bad. I then rebooted and tested with memtest86 and it couldn't even complete the scan, as there were so many errors. I replaced the ram with some that I had and retested again with no errors.

Then I rebooted the computer and wanted to work on this TB sending problem. So first thing I uninstalled Hyper-V, restarted the host machine, then went back in to reinstall Hyper-V, restarted the host machine again. After windows started, I booted the client machine, started TB and tried forwarding a message to my other Gmail. That worked, so I tried on a few more and it seems to now be acting like my other two computers with this setup. That is flawlessly.

So whether it was the bad RAM that somehow was affecting the Hyper-V install, or it was Hyper-V itself missing some component it needed which the reinstall fixed, I don't know. But thank you for suggesting recreating the profile, because that's really what made me try reinstalling Hyper-V.