Recipients getting ' 550 Invalid Recipient ' msg on reply- but alias works!

[#1]

I am able to send emails no problem - but when some friends hit 'Reply' they get a '550 Invalid Recipient' error message - but they are able to send a message to an alias address I have set-up on the orginal email address. Can anyone tell me what is going wrong and what I need to do? Thanks. Nick

[#2]

Can you post Full Headers with your e-mail address munged.

[#3]

Are you using webmail (original, not beta)?
IIRC the default 'Reply To' address is username@username.plus.com, for some strange reason.
It's been mentioned several times on the forum, but maybe it's still the same.
It can altered from 'Options' > 'Personal Information'.