Spent a while today convinced my Zoho app password was wrong. It was not.
If you set up Gmail to send from your Zoho-hosted domain, Gmail looks up your domain's MX record and helpfully prefills the SMTP server for you. For a Zoho domain it fills in mx.zoho.com.
mx.zoho.com is the INBOUND server. It receives mail for you. It does not accept outgoing mail and it never will. Gmail's error is "Couldn't connect to the server. Check the server and port number", which is accurate, but by that point most people have already retyped their password four times.
What actually works:
SMTP Server: smtp.zoho.com
Port: 465
Connection: SSL
Username: the full address, [you@yourdomain.com](mailto:you@yourdomain.com), not just "you"
Password: a Zoho app password, from Zoho, Settings, Security, App Passwords. Your normal account password will not work if you have two-factor on.
Then Zoho emails a confirmation link to the address you are adding, you click it, and you are done.
Two things worth doing while you are already in there:
In Gmail, set "Reply from the same address the message was sent to" rather than always replying from the default. Business mail then answers as the business and personal mail stays personal.
Set up DKIM in the Zoho admin console if you have not. Domains, your domain, DKIM, add a selector, put the TXT record in your DNS, hit Verify, and turn the status toggle on. Zoho does not sign your mail until that toggle is on, and unsigned mail from a new domain gets binned a lot.
Source: r/Zoho · by /u/Zito_Kadaken