Are Plusnet mail servers blacklisted?

[#1]

@Fixxxer, @Champnet The mail server has a rather short timeout after displaying the 220 response.

Once you see the 220 response try typing 'ehlo relay' (without the quotes) and if you get a list of 250 responses try the full telnet testing script here. 

220 avasout-ptp-001 smtp relay.force9.net
ehlo relay
250-avasout-ptp-001 hello [x.x.x.x], pleased to meet you
250-HELP
250-AUTH LOGIN PLAIN
250-SIZE 104857600
250-PIPELINING
250-8BITMIME
250-STARTTLS
250 OK

[#2]

@Browni thanks for the information, I'll give that a go.

[#3]

@Browni following your instructions I was able to successfully telnet and authenticate.

220 avasout-ptp-002 smtp relay.force9.net
ehlo relay
250-avasout-ptp-002 hello [X.X.X.X], pleased to meet you
250-HELP
250-AUTH LOGIN PLAIN
250-SIZE 104857600
250-PIPELINING
250-8BITMIME
250-STARTTLS
250 OK
auth login
334 VXNlcm5hbWU6
123456789123456789
334 UGFzc3dvcmQ6
123456789123456789
235 ... authentication succeeded

Upon completion of this I was then able to add one of my email addresses as a GMail send as option and verify but have not yet been able to send and will test again later.

On checking blacklisting I still found the below servers to be listed.

avasout-peh-002.plus.net
avasout-peh-003.plus.net
avasout-peh-004.plus.net

[#4]

I was able to replicate your setup and could send so the blacklist entries maybe a red herring.

For info : I have a fixed IP address in the range 212.159.x.x and use DNS servers 1.1.1.1 & 1.0.0.1

[#5]

@Champnet I agree the blacklists could be a red herring as when following the instructions provided by @Browni I was able to authenticate with the relay. I recreated the send as in Gmail and sent email successfully from all send as email addresses which were delivered successfully.

Did my tinkering fix anything?

Did Plusnet fix/change anything?

Did I just get lucky?

Either way it seems to be working for now, only time will tell if it stays this way.

Thanks to all who have contributed regarding this issue and even if it just started working of it's own accord I feel I've learnt from you and for that again I thank you.