Webmail problem

[#1]

When I send an email using Webmail it arrives at its destination showing from: username@username.plus.com
When someone replies to that email it fails, OE gives:
The following message to <xxxxxxxx@xxxxxxxx.plus.com> was undeliverable.
The reason for the problem:
5.1.0 - Unknown address error 550-'#5.1.0 Address rejected xxxxxxxx@xxxxxxxxxx.plus.com'
and Yahoo gives:
Hi. This is the qmail-send program at yahoo.com.
I'm afraid I wasn't able to deliver your message to the following addresses.
This is a permanent error; I've given up. Sorry it didn't work out.
<xxxxxxxxx@xxxxxxxx.plus.com>:
84.92.2.97 does not like recipient.
Remote host said: 550 #5.1.0 Address rejected xxxxxxxxxx@xxxxxxxxx.plus.com
Giving up on 84.92.2.97.
I have replaced my user name with xxxxxxxs in both examples.
Anyone any ideas?

[#2]

when you login to web mail you need to log in by mailbox - for example username+fred and then the email will be seen as coming from fred@username.plus.com

[#3]

Go here, click the 'Mailboxes' tab and make a note of the default mailbox address. Next login to Webmail, click 'Options', 'Personal Information', replace the email address with the one you made a note of then click 'Submit'.
Try sending and replying to an email from Webmail again and you should find all is well.

[#4]

Bob,
I thought setting up the webmail account did that automatically - certainly I don't remember doing it manually on any accounts I setup

[#5]

Yes it does but we changed the default format for email addresses a while back. During signup this now defaults to <first_name>@username.plus.com, you're also given the option to alter the local part before the '@' to something else of your choice. Catch-all is switched off by default.
Webmail on the other hand still populates the sender address as username@username.plus.com resulting in the problem being described by the OP.

[#6]

Thanks Bob (& Oldjim).
That worked 
I wpuld never have worked that out for myself