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.
Strengthen your contracts / terms of service. Make it their responsibility to configure their email systems properly so they receive emails sent from your domains.
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.