I'm developing a desktop app and it seems to work. The recipients are receiving the message, but I'm still getting this message to the sender's email account:

Your message did not reach some or all of the intended recipients.

      Subject:    Happy Birthday

      Sent: 5/25/2014 4:09 PM

The following recipient(s) cannot be reached:

      'loyd@a-lcampbell.us' on 5/25/2014 4:09 PM

            Server error: 'Invalid recipient'

That is the full message. Anyone have an idea what is happening here?

Hi,

Does this issue happen to all recipients or a certain one?

Please try to send emails to the recipient directly from Outlook (not through the desktop app), and then check if this issue continues.

In addition, please check if this KB article applies:

http://support.microsoft.com/kb/195671/en-us

Best Regards,

Steve Fan
TechNet Community Support