Mac Mail does not show recipient in sent mail

It sounds like corruption of the com.apple.mail.plist file.

If you are using any optimizers such as Mackeeper, Advanced Mac Cleaner, or Zeobit or Macpaw utility, uninstall these, as these can damage both plist and cache files of important Apple applications. No optimizers should be used with Macs. You need to keep your system less than 85% full (an arbitrary value that has been found to work best), and not use any unnecessary memory resident applications to keep the system up to speed.

Etrecheck is a good utility for finding memory resident programs.

Is it that it doesn't show any To value? Are you sending to multiple recipients? Are you using BCC?

Apple's Mail auto-replaces email addresses with contacts who match email addresses in messages in Mail. If all you are seeing is the recipients name, that's where it pulls that information.

Does the same problem happen with Thunderbird or Spark? Both are great alternative email clients.