Mail Relay Failure - no MX or A Record

[#1]

Hi. I'm having trouble with relay.plus.net, trying to set up in Thunderbird.
I followed the instructions, and sending a message says it is connected
to relay.plus.net, but then responds in a popup window:
An error occurred while sending mail. The mail server responded:  <my_username@plusdsl.net> sender rejected - invalid domain (no MX or A record).  Please verify that your email address is correct in your Mail preferences and try again.
I had this problem when I first opened the account a week ago, and some issue was found and fixed. However, now it is still there, and customer support will take 26 hours to pass it on to an expert. I have email now only because my previous provider hasn't closed his relay service. A quick google shows that this is likely to be a PlusNet DSN problem. Is anyone else getting this please.
Thanks.

[#2]

The email address you quote looks like your router PPP logon ID.
Presumably this is correct or you wouldn't have been getting as far as relay.plus.net (unless you were using some other means to connect to PN's SMTP servers)
What was the address of the email you were sending?

[#3]

plusdsl.net isn't a valid internet domain - it's just a special login used to connect to the Plusnet gateways before you can get to the internet.
The email address you specify will need to be something@username.plus.com

[#4]

Thankyou, decomplexity and jelv. I have 4 different identifiers to use with plusnet - <username>, <username>@plusdsl.net, <username>@plus.net and <username>@plus.com. So reading your advice I went through and tried them all again. (Unfortunately that is necessary as the otherwise excellent thunderbird setup instructions show the email address as
"yourname@username.plu" and the rest of the string is truncated by the window.  The panel further down just says "Username", and the instructions below the picture don't really help.)
My previous provider coped with just one string for all of these.
Anyway, for others who get stuck, the combination that worked for me is:
The Email address: <Account_Username>@plus.net
The username (in the server hostname box) <Account_Username> with no @ or anything else.