I think some of the things that could help are: * A very straightforward exit strategy: make it easy for users to at least extract their mail from your service

I think some of the things that could help are: * A very straightforward exit strategy: make it easy for users to at least extract their mail from your service, if they host it on your servers, and want to have an offline backup. * Offer an option to license for self-hosting: hosting their own email could take away their fear of uncontrollable downtime.