@Madchatthew no, you have that right. However, the MTA for postfix uses port 25 - and given that this is blocked for a month, this will mean you’ll need to relay the mail to another server or send it at the application layer to bypass it altogether.
This is a temporary measure until Hetzner open the port - and you’ll need to request that once you’ve paid the first month invoice. Postfix handles all SMTP based mail, so it’s the sending and receiving service. Dovecot is the receiver of POP, IMAP, etc so works differently, but handles mail sent to it downstream via Postfix.
All things considered, third party relay services will have a much better reputation than the IP address of your mail server so will be more reliable.