As a company we realised about 12 months ago that email can no longer be considered a reliable method of communication. I understand the reasoning but the result is the result. Many new clients we deal with have whitelists on and we have to pester our clients to add us to their lists. Not a great experience for anyone. I think we should consider email pretty much moribund at this point.
It is literally one of the last free and open channels on the Internet – I suggest you work on supporting its use and keeping it that way rather than moving to something controlled for profit or data harvesting.
It being free and open is exactly what the big providers can't stand and why they are coming up with ever more "standards" that are complex to configure as well as "reputation" scores that are hard for small senders to clear.
Also why they do non-standard stuff like accepting mail but then simply not delivering it, rather than bouncing it.
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There is no such thing as a free and open channel on the internet that will not suffer the tragedy of the commons.
Calling it a "free and open channel" when 80% of consumers are on Microsoft/Google addresses that control who can deliver e-mail is laughable. It's dead, Jim.
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It can be reliable, but requires an explicit agreement between the two parties. When my server connects to your server and sends a message, that part is reliable enough. If your server decides to delete the message after receiving it, that's entirely on you. You could configure it to never delete messages from me and then it's fine. The problem is that in reality nobody does this, so your vendors and customers get treated identically to random spammers.
Strengthen your contracts / terms of service. Make it their responsibility to configure their email systems properly so they receive emails sent from your domains.