How can Isubmit Proposal

I am having challenge submitting Business proposition to Sponsorshipandpromotions@mtn.com error is rejection of my email by postmaster my email address is: bridgecitymulticoop@gmail.com below is given error message.

Sponsorshipandpromot. . .
Sender Action Required

Sender not allowed

How to Fix It
Try one or more of the following:

If you're a member of the group, you might be registered with an email address other than bridgecitymulticoop@gmail.com. Resend your message using the email address where you received a welcome message from the group.
Ask the group owner to add bridgecitymulticoop@gmail.com as a group member. If you don't know who the group owner is, contact the organization's customer service department for assistance.
Ask the group owner to allow people outside their organization to send email to the group.