I remember I had to go through a couple of hoops with outlook.com and gmail(?) using some un-/under-documented web sites to get "approved" (sending a mail, getting a reply, typing reply in web form).

And in my experience the major services have unpredictable spam filter behavior.

I even saw Google being cute and assuming mail in Japanese was spam - because my profile is western.

So yeah. If your goal is 100% (not high 90s) deliverability - including the odd big attachement or foreign encoding mail - it's not trivial.

Thankfully I don't care that much if a recepient at Gmail or outlook don't get my mail;it's their fault. If it's a problem the recipient should move to a more reliable mail provider.

It's obviously not as easy for commercial/work mail...