I'm having a problem with outlook 2007,an outside person will email a person in the office,the person will then email me,I need to respond to the outside person so I click on the link with that mailto hyperlink to get the email address.I respond and send the email but it keeps on bouncing back,its happening to a lot of email links lately.Is it a setting on my side?it can't be the outside persons email account otherwise they wouldn't have been able to send the email in the first place.

Any ideas where I can look at settings etc?

Any error messages being received ?
They are always a good place to start.

Have you tried mailing your own personal external account ?
Good way to check external connectivity..

I have emailed a few external accounts and it works,its only selected ones tha don't work,very weird.
The error messages that read when its says message undelivered are either the inbox is too full or it just says it wasn't delivered.The funny thing is that the inbox only has about 50 messages.I have attached the 2 failed messages that show.

Well the first error message indicates that the person is no longer on that address.
Could be a simple spelling issue or it's been deleted.

The second, assuming it only has very little messages in it and is below it's allocated space, would indicate a corrupted mail box. This would need repairing.

I was getting to the conclusion of a corrupt mailbox.

I understand what you saying about the mailbox not existing but an email was sent from that email address,then that email obviously has the address in it so I just copy and paste.

How would I go about repairing the mailbox?

But if the mailbox is corrupt,will it not then not work at all as opposed to working with selscted mail addresses?

Any idea what I can do or look at?

How can I go about repairing the mailbox?

You'll need to use the Office Diagnostics tool.
There's details here: How to Fix Microsoft Office Problems with MS Office Diagnostics

Sorry I can't be of much more help, but I work with Lotus Notes and not exchange e-mail.