I've never heard of them before, but they look like they're trying to beat Proton at their own game (including by providing a bring-your-own-domain option for free in order to seduce current Proton customers).

On the other hand, people have claimed Aster is "vibe coded to hell" after another random competitor pointed out a bunch of issues that seem to have been caught from a Claude Code audit, so maybe this isn't the most serious product when the people behind it don't really seem to know what they're doing (like creating and then apparently immediately deleting keys?!)

Edit: from the same thread, their CMO allegedly sells Roblox exploits. This isn't the kind of background that causes me to put faith in a company, that's for sure.

I tried it out for a month. I just had unimportant things going there. It seemed alright but, I don't code or anything so I can't speak to how well all that worked out. I will say that my account did get deleted for inactivity so that's a thing to be aware of. I know some people were critical at one point of Tuta for that.

I had issues using AsterMail. Couldn't log in multiple times. Support seemed to only be through Discord. Some emails refused to delete, only disappearing for a second and reappearing. Maybe it's better now, but I'm happy with Proton. It just works (and I like their filtering UI).

One test I'd apply to any new private-mail provider before moving anything important: how easy is it to leave? Check whether you can export the whole mailbox in a standard format (mbox/EML), whether there's IMAP/SMTP access or a bridge, and above all use your own domain. With your own domain, if the provider gets bought, enshittifies, or deletes your account for inactivity like someone mentioned here, you change the MX records and you're gone in an afternoon.

Given how young it is and the security disclosure back-and-forth above, I'd keep it to low-stakes mail until it has a track record and an independent audit.