It may also depend on your email service.

With the minimal "free" Hotmail accounts, as an example, any attempt to send more than 10 messages the same day may cause them to assume you're sending spam, and they'll block all outgoing from your account for a few hours (or at least that's supposedly what happened to me once a few months ago). ¹My account was not one they were supposed to do that with, but they f$#%@ up. Their explanation was a little unclear, since I don't speak fluent weasel, but that's the gist of what I was told.

¹ On hotmail accounts where they were supposed to be applying that rule you generally can't use OE, since they're "webmail only" accounts. Different rules, also mostly unpublished, seem to apply to the extra cost accounts and/or "charter accounts".

They apparently were (are?) really inconsistent about when/if they apply this and other similar "rules," so confusion is to be expected.

I have heard of other email services applying similar rules, such as vague limits on maximum number of addressees (CC or BCC) on each transmission.

Policies of this sort can and do shift about frequently, and are seldom published to users when changes are made; but it's supposedly all to help cut down the spam.

John