That should work. However you could try reverting the outgoing server to relay.plus.net (assuming you are a Plusnet customer) but change to port 587 instead of port 25. You will need to select authentication and enter your main account username and password (or username+mailboxname and mailbox password) for authentication. Note this is authentication but without security (SSL/TLS). You can use the same setting (port 587 and authentication) when connected via the Plusnet network.