I should be able to help you with this. I setup and managed zimbra mail services on Linux, also worked with exchange and office365. Most often this is a config Issue
Relevant Skills and Experience
Postfix SMTP, Symantec Messaging Gateway, DNS, SPFs,DKIM
Proposed Milestones
$5 USD - Examine Mail Headers of Mails you send
$10 USD - Verify your A and PTR Records and correct if necessary
$10 USD - Verify Your SPF Records and DKIM keys if existing
Additional Services Offered
$50 USD - Setup SMTP servers for distributed load
$100 USD - SPF setup
When did the issue start and what changes were made at this period