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osx mail

Mail > Preferences… > Server Settings

osx mail - accounts - server settings - edit smtp

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

osx mail - accounts - server settings - 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]

osx mail - accounts - server settings

Outgoing Mail Server (SMTP)

Account: rsxxx.realsender.com

[Save]