IPhone 4 email mystery

[#1]

Hi folks. I've just switched to Plusnet and have amended all the settings on my Iphone4 . It works fine, receives and sends e-mail on wifi and my O2 3G connection.
So, I made the same changes to my wife's iPhone4, where the only difference in settings is that her username is in the Plusnet format for a subsidiary address set up under the main account. Her iPhone email now works perfectly on our wifi connection, and those same settings work fine on her wifi only iPad too.
However, when she tries to send e-mail using the O2 connection, she is told that the recipient's email address was "rejected by the server" . She can receive email on the O2 connection, but not send.
This has me baffled, because both my iPhone and hers have exactly the same server detail entered, mine works, hers doesn't.
Anyone want to suggest what might be causing this?
Thanks.
Tom

[#2]

Authentication must have been set for sending from a mobile connection. Has that been set up on your wife's phone using your account username and password or the username+mailboxname and password used to collect her incoming mail (which works)? If it's the latter I suggest trying the account username and password.

[#3]

Sometimes the iPhones get the email profiles garbled.  All looks OK but it fails as you describe.  My wife's phone had exactly the same problem - several PN accounts work OK in all locations, however one did not.  No matter what I did with the configuration, it would not work  In the end I deleted the mail configuration on the iPhone, rebuilt it and all was fine.
There is another possibility - what port number have you set for the SMTP relay.plus.net server?  To be pedantic, port 587 is the correct one for cross network (authenticated) SMTP connection.
I have also noticed that in some locations EE (Orange) phones will not send email via the relay service.

[#4]

There's also been some instances where the PN mail servers have been rejecting smtp (even authorised) because of 'poor ip reputation' . Here's one http://community.plus.net/forum/index.php/topic,119470.0.html