Email Authentication is Failing

Email Authentication is Failing

I'm trying to setup gitlab with email authentication. I used the following configs picked up from: https://docs.gitlab.com/omnibus/settings/smtp/

gitlab_rails['smtp_enable'] = true
gitlab_rails['smtp_address'] = "smtp.zoho.com"
gitlab_rails['smtp_port'] = 587
gitlab_rails['smtp_authentication'] = "plain"
gitlab_rails['smtp_enable_starttls_auto'] = true
gitlab_rails['smtp_user_name'] = "admin@jambusys.com"
gitlab_rails['smtp_password'] = "------------"
gitlab_rails['smtp_domain'] = "smtp.zoho.com"
gitlab_rails['gitlab_email_from'] = 'admin@jambusys.com'
gitlab_rails['gitlab_email_reply_to'] = 'noreply@jambusys.com'


SPF, DKIM, DMARC are all set correctly, and verified. 

But still email sending fails from the giltlab server due to SMTP connection error.

gitlab(prod)> Notify.test_email('admin@jambusys.com', 'Message Subject', 'Message Body').deliver_now
app/mailers/application_mailer.rb:14:in `block in <class:ApplicationMailer>': ApplicationMailer::SMTPConnectionError (ApplicationMailer::SMTPConnectionError)

Can you guide me about what is wrong here and how to get past this?