Tabbing between fields in a new message in Apple Mail - Mojave

Thanks for the suggestion, but actually, it was a setting under "Keyboard" that got turned off when I upgraded (I couldn't do a "migration," due to the nature of the upgrade — long story, involving multiple senior Apple support angels). See bottom of screen shot below: "Full Keyboard Access."

(P.S. to Apple: this is why I HATE "upgrading" to a new OS (MacOS or iOS). No matter what, it seems there's always hassles . . . it's like someone coming in to rearrange my office furniture with helpful new "features" (and bugs) that end up taking more time than it's worth. My computer is not a toy, it's a business tool that I don't like being mucked around with when I finally get things working on it. Sigh!)

Well, the screenshot didn't get included, and I couldn't figure out how to edit the last entry. Setting is at the bottom of the dialogue box under "System Preferences — Keyboard — Shortcuts."

David