I accidently deleted an email and am trying without success to retrieve it.

Try logging into the Web Interface of your Mail Server and seeing if it may still be there

MichaelMcPherson wrote:

I accidently deleted an email and am trying without success to retrieve it.

Look in your mail Trash folder.

If you don't see it there make changes to you Mail>Preferences for future faux pas

I'm afraid TM will be little help to retrieve a specific email. That went away starting with Catalina. Like suggested perhaps it remains on the server by your email provider.

I have been known to send an email to known recipients and ask them to send back an accidentally deleted one. I've been kidded, but they did as I asked; we all make mistakes. If to a business, that may be a lost cause.

Maybe this will help: recover deleted email with time machine - Apple Community

How do I log into Web Interface?

That will depend on who your Mail Account is with ?

iCloud Mail is = www.icloud.com