How do I locate specific sent emails in the 'Sent Mail' folder?

You might try rebuilding the Sent Mailbox.

Create or delete mailboxes in Mail on Mac - Apple Support explains how to rebuild Mailboxes.

Excerpt:

For example, you may need to rebuild if messages appear to be missing or garbled, or if you can’t find all relevant messages when you search using the Entire Message search option.

You can rebuild any Mailbox, or all Mailboxes.

Not enabling, how?

You didn't provide any details on what you're doing, or what you're seeing, so it's hard to be specific. All I can do is give you pointers..

First off... how are you searching for these emails?

You should be using the search field at the top of the Mail window.

Then you need to look at which mail folder you have selected in the sidebar - if your Inbox is selected, then it's only going to show you messages in the InBox, which likely won't include messages you've sent out.

So, you may need to select the Sent mail folder for your account, or choose 'All Mailboxes' in the sidebar.

Additionally, Mail.app uses a broad search criteria, so it will search for whatever your type through every message - addressees, message body, attachments, etc.

If you're looking for a specific email address you can prepend 'To:' to the email address to find messages addressed to that person, e.g.:

To:[email protected]

This will limit the search to just the To: field.

If none of the above helps, please post back with more details of what you're trying and what you're experiencing. It might shed extra light on the problem.

Thanks so much for reaching out. I have been trying to search specific emails--from a specific sender. This always worked for me before. It's suddenly stopped working. I have had to go through emails to find emails sent or received. Thank you for your response.