1. First, try to find if the sender's domain is blacklisted. You can check your domain status by these links:
2. If the domain is blacklisted by any of these blacklist sources, our email system may just block the email according to list in order to secure our users. Try to contact senders to fix their domain issue.
3. If the domain is not blacklisted, sender may get the bounced back message, it usually contains a reason why the email is rejected. (For example: no such mailbox user, user mailbox is full, mail is reached it's maximum size, unaccepted attachment type...)
4. Misconfigured SPF record - I.T. administrator may only allow email can only send via designated email servers. For example, domain exampledomain.com may only allow email users to send email via exampledomain.com and mx.exampledomain.com mail servers. If a sender send email via other email server (e.g. another-email-server.com), our system will reject the email as it's violated their SPD record (SPF validation failed).
If this is the case, you may try to contact the sender to ask them to review their SPF record, or re-send email via their designated server(s).
You may submit a support ticket to us and request a review if your problem can't be solved by yourself.