Messages are not being received or sent to spam folder

Hello!

I have been using mailbox.org for 5+ years. Recently, I noticed people not receiving my emails or the emails going straight to the spam folder. In one case, I sent an email to someone using gmail, and my email went directly to their spam folder. In another instance, I sent an email, got no reply, sent the same email from a backup gmail account, and got an immediate auto-reply. This indicated, to me, that it wasn't received at all by the recipient when sent from my mailbox.org account.

Is there something I can do on my side to fix this? I know that new accounts can have low reputation and might get rejected, but this is an email address I've been using for years.

Thank you.