Is this happening to all the emails you try to send?
If you got to the Gmail web site, can you send messages from that account without any problems?
Have you tried deleting the account and adding the account back?
Are you running any anti - virus programs? Using a VPN?
Try booting into the Safe Mode and see if the same thing happens.
Safe Mode - About.
If it works in the Safe Mode, try running this program when booted normally and then copy and paste the output in a reply. The program was created by Etresoft, a frequent contributor. Please use copy and paste as screen shots can be hard to read. Click “Share Report” button in the toolbar, select “Copy to Clipboard” and then paste into a reply. This will show what is running on your computer. No personal information is shown. If the log won’t post, try posting it in Pastebin and provide a link in a reply. After pasting the report in a PasteBin page, go to the top of the page, and copy the address in the URL bar. Paste that in a new reply. Pastebin.
Etrecheck – System Information.
Web site version if required. https://etrecheck.com