The following info is from a Rackspace doc:
Update the Password Authentication Method for Apple Mail

For this error:
Connections to host secure.emailsrvr.com on the default ports failed.
proceed as follows:

To configure the settings for Apple Mail correctly, perform the following steps:

• In Apple Mail, go to Mail->Preferences.
• Click Accounts, then click the Server Settings tab.
• Click Outgoing Mail Server and select Edit SMTP Server List.
• Click the Server Settings tab.
• For Authentication, select Password rather than MD5 Challenge-Response.
• Click OK to close the SMTP window and click OK again to close Preferences.

For this error:
Connections to host secure.emailsrvr.com on the default ports failed.
proceed as follows. 

• To configure the settings for Apple Mail correctly, perform the following steps:
• In Apple Mail, go to Mail->Preferences.
• Click Accounts, then click the Server Settings tab.
• Click Outgoing Mail Server and select Edit SMTP Server List (may need to scroll down to bottom on drop-down menu)
• Click the Server Settings tab.
• For Authentication, select Password rather than MD5 Challenge-Response. (if you do not see Authentication, uncheck Automatically manage connection settings)
• Click OK to close the SMTP window and click OK again to close Preferences (or red dot on top left corner of window).

Additional Resources

When You Can’t Send Mail: Fixing SMTP Problems

Use Connection Doctor in Mail on Mac

• In the Mail app on your Mac, choose Window > Connection Doctor.
• Review the status information. …
• When you're done, click Check Again to verify if your actions resolved the problem.

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