iCloud account or another email provider? Is this happening in multiple accounts? Have you tried quitting Mail and reopening it? Are you running any anti - virus programs? Using a VPN? Have you tried the Safe Mode to see if there is a difference?
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