You aren't selling to techies. This is super easy to set up for almost any engineer (beyond a few fantastic features you added).
What you need to do is effectively twist the knife, make the reader feel the pain. "Don't you think $10 / mo is worth your sanity and a clean inbox?" essentially.
$10 is basically nothing to your modern professional. That's 1 - 2 coffee runs.
$1 or free (!!!) is way too cheap for an indie maker. Ignore the people who complain about price. There are always going to be people complaining, fire them. Those types will always find something to complain and twist your arm about.
Even $10 / mo for me, someone who has set this up myself, might be worth it just for additional features and being able to easily add more domains.
EDIT: Save free forever for the VC-backed companies who are burning other people's money in the hopes of hockey stick growth.
Free trials are fantastic, but free forever is a losing gambit in my opinion.
That makes sense. You (and your customers) definitely know the market, so data-backed decisions are best. Could also be an A/B opportunity for the future. Good luck, the product looks great!
My main concern would be the fact that complete email solutions such as Tutanota, Posteo are very cheap at 1 euro a month or so.
So I'm not sure users who aren't as concerned about their privacy would think it was worth it for $4.99 per month+
I mean I could be completely wrong, just my thoughts from some of the feedback I've received so far.