Both. The greeting is a separate entity and despite ending in a comma or semicolon is not part of the first sentence of the body of the email. This is inherited from English standards for writing letters on actual paper. Unless it's being graded for a class in school, is part of some official communication, or is going to a copy editor, though, it probably doesn't actually bother anyone if this convention is flouted.

"Dear Ms. Smith,

We would like to inform you..."

vs

" Dear Ms. Smith,

we would like to inform you..."

But if you write it on one line, the capitalization of We is definitely wrong.

However you can always tell your correspondent is German by their capitalization of second-person pronouns! Also the awkward use of "kind regards" because they can sense that "with friendly greetings" doesn't sound right.

"Dear Ms. Smith, we would like to inform You that Your cloud subscription will soon be expiring. Please send Your payment by registered letter to..... Kind Regards, Dieter" ;-)

Thanks. I haven't been taught this rule but my brain seems to associate the capitalization requirement with the newline rather than the salutation itself.

Mail in iOS capitalises it automatically, which drives me nuts when writing in German. (Look, it's not a new sentence, why should I capitalise it?)

My logic/rationalization is that even though there's no period after the "Dear,\n", the newline makes it a separate paragraph and therefore a separate sentence.