Unable to use gmail through Zoho

Unable to use gmail through Zoho

I am running my gmail email account through Zoho, and while I can receive emails, I can't send or reply to any. The following error appears:

Unable to send message;Reason:530 5.7.0 Must issue a STARTTLS command first. li15sm44069707pab.2 - gsmt

Does anyone know how to rectify this? My colleagues also use gmail through Zoho and do not experience any problems.

Thanks.