When you use passbolt to send an email via your Titan mail account, it then turns and sends it to the receiving mail service. If the receiving mail service is saying 550, then passbolt gets a 550.
The test email goes to no-reply@passbolt as a way to verify your SMTP settings without complicating the matter. However, in practice the end receiver of emails may be stricter.
If you are not able to send a verification email to your account at Titan, what is the result of the test command when you do that?
See this discussion for how the test email (at least in the past) was different than actual emails. This is the intent of my questions.
The good news is that the issues are all resolvable. But it’s important to understand that the passbolt app has no control (obviously) over external mail provider settings or requirements.