Can't send through SMTP using gmail

For the past few years I have used Zoho to host my email, and used gmail as a client, sending from gmail "send as" my zoho-hosted email address . However, today I started receiving an error message when sending through gmail, saying that "

Message not delivered

You're sending this from a different address or alias using the 'Send mail as' feature. The settings for your 'Send mail as' account are misconfigured or out of date. Check those settings and try resending."

I tried to delete the SMTP setting on gmail and reset it again, but every time I do, I get an authentication error on gmail, using the Zoho recommended SMTP server smtppro.zoho.com or smtp.zoho.com, and the recommended SSL/port .

Any idea what could have gone wrong?

The only change I have made is that a couple of days ago I enabled two step authentication on that zoho account, but I have reversed that today and still no luck.

Thanks a lot for any help!!!