I can chime in on this.
We ARE a non profit, run our own server, and in addition provide email service to a number of other non profits. @Douglas comments above are the primary selling point, and we can in fact provide 'proof of concept' on it. Many smaller non profits are not familiar with what an "acceptable use policy" is for email. We have built some and help them to understand that once you start using gmail or another provider for your email, you are at their mercy. If you accidentally delete everything from an email box, they will decide if they will give you the backup data or not. If your account gets hacked, and they cant prove if you are really you or not (since you can not call gmail on the phone and talk to a tech) they may decide the best thing to do (for THEIR protection, not yours) is to just kill your account. If you ahve some legal sh** going down and need to terminate emplotyees, volunteers or executives, you need to have absolute control over the email data, and if you are using gmail or others, you do NOT have that.
Back a few years ago, An ED of one of the non profits was terminated by her board. We are the non profits email host. The New ED asked me to look at something because she could not find some important emails she was expecting. Turned out the previous ED set up her domain email account to forward everything over to her personal Gmail account, and then delete everything from the non profits domain. The real problem was that it was close to a month before they informed us of the leadership change. So this forwarding and deleting had been going on for weeks after her termination. The new ED had no information the state or the AG's office was sending them as it was all being forwarded to the old ED's personal Gmail account. Gmail would not give us access to the info because it was the previous ED's personal email account. We ended up having to look at server logs and see what was inbound to the previous ED's mailbox and then contact the senders to ask them to resend info,
We have also had volunteers or staff go rogue from various non profits - where they start sending (forwarding) everything to their personal email (gmail, yahoo, etc) account and then reply to the sender from their persoanl account, and not the non profits domain account. non of the conversations or data is captured through smartermail or backups, and if the staffer or volutneer quits or is terminated, you have no access to any of that info.
Other situations i have seen is that some volunteer set up a "donationsForOrgName@gmail.com" account and started collecting donations. Meanwhile the non profit has no control over that data, or even the money coming in for that fact. This of course comes under an acceptable use policy and a strict enforcement of using only domain based email for org activity. But is a prime example of how the big tech providers will not give you access to the account info, even if your name is literally part of the email address.
I could give you a list of reasons why as a non profit it is better to use smartermail than gmail. Most of them revolve around data ownership and control of the account. "FREE" sounds awesome...until you need to go and get immediate access to info (and are legally responsible for) that you do not have direct control over. Then you quickly learn how truly expensive the decision was to go for the "free" offering.
Regarding SPAM :
I posted about the ownership part of this, back in 2015 :
Another idea in 2017 :
Another suggestion in 2018 :