user will received this error message from email client when try to send email.
"Cannot Send Mail The recipient “firstname.lastname@example.org” was rejected by the server"
Email users using ipad/iphone/androind
Then you most probably did not set SMTP authentication when the email account was configured on that specific device. This is not only the user’s fault, Apple states in the configuration steps that these credentials are optional, well, they are not really optional, unless the SMTP server you are using is Open Relay.
Access your Settings icon > Mail, Contacts, Calendars > Email account > Account > “Outgoing Mail Server” | SMTP > Primary Server
Here you will have to complete the following fields:
Password: ***** (your email password will go here)
Authentication [usually here you select Password]
Please enable SSL
Next, in order to save these settings, tap on “Done” in the upper-right corner. Now you should be set up and ready to send some emails.