Help - "535 Authentication Failed , code: 535"

Help - "535 Authentication Failed , code: 535"

Hi Zoho support team,

Today, all of a sudden, my Zoho account stopped redirecting emails to my gmail account. This means,  Zoho was configured to redirect the emails I get there to my Gmail account. It had been working well but now I keep receiving messages from Mail Delivery Subsystem after I send an email.

Also, when I go to 'accounts and imports' in Gmail, zoho mail is correctly selected as the predetermined email address. And then, when I go to 'edit information' and try to confirm my password, the system throws this message at me: 

"535 Authentication Failed , code: 535"

But I'm sure the password I'm using is correct. 

I depend on the solution to the connection between zoho and gmail, since its my working tool. ¿Can you please guide me to solve this problem?

Thanks!!

Víctor.