Server Rejecting Mail from New iPhone

Server Rejecting Mail from New iPhone

Attempting to send email from my new iPhone, it lands in the Outbox, with a message saying "The sender address was rejected by the server."

As far as I can tell, the account is set up correctly.  I'm receiving email OK.  The outbound server is SMTP.zoho.com

Edit: this is for two active email accounts.  They work fine on my two Macs.  Settings are the same on the iPhone.  This is an iPhone 5, running iOS 6.1.4.

/Mr Lynn