Recently i came across an issue with unable to send emails externally to our domain from IMAP/SMTP clients and found an solution for the same

Hope this would be helpful for others if similar issue occurs

Issue: Unable to send mails from IMAP/SMTP clients.
Error: 5.7.1 The client does not have permission to send as this sender.
Resolution: The sender specified in the MAIL FROM field of the SMTP protocol conversation is an address in an authoritative domain.
However, the session doesn’t have the ms-Exch-SMTP-Accept-Authoritative-Domain-Sender permission. This might occur if a message was submitted from the Internet to an Edge Transport server from a sender address for which the Exchange organization is authoritative
 Shell Command#
Get-ReceiveConnector “Default SERVER01” | Add-ADPermission -User “NT AUTHORITY\ANONYMOUS LOGON” -ExtendedRights “ms-Exch-SMTP-Accept-Authoritative-Domain”
Get-ReceiveConnector “Default SERVER01” | Add-ADPermission -User “NT AUTHORITY\AUTHENTICATED USERS” -ExtendedRights “ms-Exch-SMTP-Accept-Authoritative-Domain”

A related article which talks about the permission and error:

Exchange 2007 Transport Permissions Model
http://technet.microsoft.com/en-us/library/aa997170(EXCHG.80).aspx