"Mass Email" from Leads reached recipients' SPAM folder

"Mass Email" from Leads reached recipients' SPAM folder

I'm new to zoho CRM. I specifically upgraded zoho CRM to access the mass email function. 

Unfortunately, when I saw zero replies, I asked one of my recipients to check their inbox. They reported the email went into the SPAM folder.

Please advise on what I can do as obviously this is really important. 

The only thread I could find was here, but there was never an official response. 

Many thanks in advance.