The thing I'd look for is encryption by default. I glanced at your link and it did not seem to be encrypted by default, so I don't like it already. 🤔 Also they block Tor traffic from even seeing their site, kinda annoying ehen Proton and Tuta both allow Tor traffic, I had to use a non-Tor browser (over VPN) to access it.
I can't seem to find a Mailbox.org client on F-Droid, only Tuta is on F-Droid. So I personally choose Tuta with custom domains. You'll want custom domains to be able to mvoe providers easily.
Wtf? Then why did PrivacyGuides.org recommended it? 🤔 I didn't look closely becuase Tuta had a F-Droid client so I just went Tuta (I also heard about them years ago), didn't look at Mailbox.org.
Mailbox.org works with IMAP so you can use a regular email program. The mailbox is not encrypted by default but I saw that there's an option to enable it involving PGP keys.
From what I see here it can automatically encrypt incoming emails with PGP, which I know Fastmail doesn't have this, and the advantage would be that you get security similar to Protonmail but you're not locked into their clients.
AFAIK, Mailbox.org was once open source, but has not very recently become closed source.
PrivacyGuides.org recommends it, as does Tresorit (encrypted cloud storage), for example, which is closed source and is one of the services I use.
Providers that implement encryption, have been audited, have been on the market for years, and have a clean track record of security or privacy scandals, which there aren't many of these types of services these days.
But I personally don't like them and try my best to avoid closed source softwares as well.
In conclusion, PrivacyGuides mostly recommends open source software but also recommends some that are not based on their features, reputation, security and maturity.