"Send As Denied" Error emailing from SRM

When sending email from SRM the error "Unable to send email" is displayed, and the email error log states "Submission Exception: Send As Denied"

These error messages are typically encountered with an Office 365 email integration, but are not exclusive to them.  The ability to send email from SRM relies on a secure SMTP connection to a host email server, typically via authenticating user/email and password.  By using that account, all email sent from SRM is routed or relayed through that account to be sent to the external recipient.

By default Office 365 does not allow this behavior, and must be configured to allow email relay so all operators are able to send email through the authenticating email account.  The following links can be useful for setting up this configuration:

Give mailbox permissions to another user

Send As Denied Error SMTP Setup