This basically doesn't work because it would require that:
- your ISP authorizes such port
- and ALL target MTA enable such port too.
Reason why you are blocked - assuming our analysis is correct - is that your provider blocks outgoing flow on port 25.
Trying to workaround using another port works only for servers using such port. Unfortunately, your provider blocks port 25 because this is the standard port for SMTP
Another aspect to be taken in account: port 587 is used from MUA connecting to MTA. Between MTAs, port 25 is used first and then TLS might be activated, for what I understand.
Is there any reason why you don't want to enable smart relay