Is it possible to manage multiple external email accounts in Zoho Mail?

Is it possible to manage multiple external email accounts in Zoho Mail?

I have started with Zoho Mail recently (new domain and email) but also have two external email addresses that I would like to manage using Zoho Mail.  One is Gmail and one is Yahoo.  I have tried over and over to get Zoho to link with Gmail through the "Add POP3 Account" feature in Settings.  I have used all of Gmail's required settings and it will not authenticate.  Any help would be great.

kBrooks