Port 25 & lack of SMTP authentication gets me blacklisted

[#1]

I'm new to PlusNet and had difficulty setting up my outgoing emails. I tried to use basic password authentication, but my password wasn't recognised. I now have it set up on relay.plus.net with no authentication and it seems to work OK.
In the past, with my previous ISP, I was often blacklisted because many blacklisting services don't like unauthenticated SMTP connections nor port 25. So I used authentication and port 587, but there doesn't seem to be this option with PlusNet.
Is there anything I can do about it? My work relies on sending out bulk (i.e. around 100) emails every now and then.
Chris

[#2]

This is perfectly fine and will allow you to send normally.
Although our relay service is not intended for the use of bulk mailing, 100 or so messages several times a month will be fine. Even a residential customer will generate more than this on occasion.
Authentication to our relay whilst you are making use our our broadband service is based on the source connection. i.e. provided you are using the broadband, relay will accept the connection.
You can make use of authenticated SMTP, however this is restricted to your primary account username and password. Unfortunately this cannot be a mailbox username and password.