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It's not a mobile constraint. Nearly every desktop mail client also uses a single To line.

I mentioned mobile because as far as discoverability is concerned, there is an order of magnitude more mobile users than desktop users, so the vast majority of users will have no problem understanding this concept.

And if you've been a desktop user long enough that you havent used a smartphone then you're probably used enough software that have similar patterns (and used email enoguh) that you shouldnt have trouble realizing that single To line means you can add multiple recipients to the same line.