Mac OS/X Mail

Mail > Preferences… > Server Settings

Outgoing Mail Server (SMTP) > Edit SMTP Server List …

[+] Create an account

Description: rsxxx.realsender.com

User name: (the one we sent you)
Password: (the one we sent you)

Host Name: rsxxx.realsender.com
[  ] Automatically detect and maintain account settings

Port: 587    [x] Use TLS/SSL
Authentication: Password

[OK]

Outgoing Mail Server (SMTP)

Account: rsxxx.realsender.com

[Save]