Attempting to set up GMail "Send as..."

Attempting to set up GMail "Send as..."

I'm trying to set up "Send as..." in GMail so that I can use GMail to manage my domain account (which is arranged through Zoho) without having to change logins or enter Zoho's website. I want to manage everything from within GMail. However, with these settings, I'm getting an error message:

SMTP: mx.zoho.com
Port: 587
Username: me@mydomain.com
Password: (correct)
Secured w/: TLS

The error is: Couldn't reach server. Please double-check the server and port number.

Has anyone an idea of how to fix this? Thank you!