This is (IMO) a misinformed approach on their part due to the absolute mess around port 465.
Port 465 (SMTPS) is VERY frequently blocked in VPS environments as an anti-spam measure as it’s the historic port used for MTA-MTA TLS communication. Nowadays it’s been reassigned to SUBMISSIONS (SUBMISSION + implicit TLS) but I don’t anticipate VPS providers ever unblocking it due to the fact there will always be MTAs listening for MTA-MTA traffic on this port.
Port 587 (SUBMISSION) is explicitly the MUA-MTA mail submission port and is what should be used for authenticated mail submission in combination with STARTTLS.
Even so, port 587 is still sometimes blocked by ill-informed VPS providers.
Notwithstanding my rant about this, I see you can connect to that port, so what happens if you manually send mail from the VPS and from the container via port 465?