They’re trustworthy in that they are good people (source: I used to work there) and that they won’t go out of business anytime soon. They have great customer support. You can trust them in that they are a reputable company.
If your question is more along the lines of “are they extremely focused on privacy?” then I would say no. Afaik, they don’t take any special measures to have your email be encrypted at rest, or support special E2EE features. That said, email is inherently unsafe by design so your best bet is to either use PGP or use a different method of communication, like Signal.
If you’re looking for more than that, then go for more privacy-focused alternatives like Tuta, Proton, Posteo, Mailfence (also in Belgium) or Mailbox.org.
I would happily consider Posteo but the fact that they don’t support custom domains is a deal breaker for me. That said, using an email aliasing service in front of it could be a solution.
If - for any reason - I want to move email providers, I don’t want to change my email everywhere.