People here are speaking about how gmail will "effectively spambox your mails by default". That's not been my experience (from setting up multiple small customers). Anyway, at least I've never heard of gmail just eating your e-mails. They either get rejected or accepted and put somewhere (maybe the spam folder, but at least somewhere).
Office365 / hosted Exchange / Outlook Protection or whatever is it called these days... they should be routed to /dev/null by everyone.
They just won't track your reputation unless you send them more than 100 mails per day. Does this sound bad? There's more: if they don't have reputation info from you (because they refuse to track it due to the low volume), your mails will go to spam inboxes even when their filters indicate that the message is not spam.
And there's more! Don't dare to ever get a bad reputation (i.e.: a user managing to get hacked and their account used to send a couple hundred spam e-mails before triggering your countermeasures). If this happens you are 100% fucked. Now they will DROP your e-mails. Hear, hear: their servers will accept your mails and just DELETE them. No spam folder, nothing.
You will try every possible thing: set up everything for their feedback loop, sign up in their "Smart Network Data Services" to track your reputation (it will be empty except for that day)... and finally contacting them at their sender support.
Do you want to know what they will reply? That you should be patient and let your reputation build up over time. What a joke! How on earth can your reputation improve when users cannot mark your mails as "not spam" because they (outlook, not the users) are simply DELETING them without a trace?
Oh, there's a way out of all this though: obtain a "Return Path Certification" [1]. That is, pay them an absurd amount of money and your mails are guaranteed to get to the users' inbox unless you are clearly spamming (all of the above assumed you are NOT).
Up to this final point you could think they just do their best and all that I've explained is collateral damage. That last "pay and we let you off the hook" is what clearly signals to me that this is an elaborate scheme to get small players to either pay them anyway or just give up and use a big-provider service.
[1] https://sendersupport.olc.protection.outlook.com/pm/services...
