I'll add in a vote for IMAP support. I am anti-Google in almost all regards. Currently I use FastMail as my email provider, and Mail.app as my client. I pay for FastMail, and similarly I would very gladly pay for a good cross-platform mail client. I'd sign up for a yearly subscription, even!
I do recognize that IMAP is not as rich a data source as GMails API, but I'd encourage you to not feel limited by that. I don't mind using hosted email, as long as the revenue source is coming from my wallet, rather than ads.
I even had spec'd out building an OSS self-hosted server that ingested email via IMAP and then stored metadata in its own database - this had the benefit of being as rich as GMail APIs, but me as a user still getting to own my data.
I do recognize that IMAP is not as rich a data source as GMails API, but I'd encourage you to not feel limited by that. I don't mind using hosted email, as long as the revenue source is coming from my wallet, rather than ads.
I even had spec'd out building an OSS self-hosted server that ingested email via IMAP and then stored metadata in its own database - this had the benefit of being as rich as GMail APIs, but me as a user still getting to own my data.