Cannot connect to SMTP server using TLS at port 465


I’m working with an email client that needs to relay an email through HubSpot’s SMTP server. This client apparently only supports TLS, not STARTTLS, and it hasn’t been able to get a connection to HubSpot’s port 465, which is listed as the TLS port in the product documentation. I have tried to telnet to at port 465. It does connect, but anything I enter, including just “EHLO”, causes it to disconnect immediately. I’ve also tried several other email clients, and while they have no trouble sending an email through port 587 (STARTTLS), none of them have been able to send one through port 465 (TLS). Is there some configuration I’m missing? Email protocols are not particularly in my wheelhouse, so I might be missing something obvious.