You may wish to check here to check on your email client configuration.
If the issue still persist, do let us know the IP address of the client that is having the issue by visiting https://whatismyipaddress.com and the following details. Also, if you have receive any bounced email that forbids you to send out emails, do forward the email to [email protected] , so that we can further assist.
1) Sender email address
2) Recipient email address
3) Approximate Date and Time of the email sent
4) Subject of the email