Overview

When an email is being forwarded from an external domain to a Kerio Connect domain, the sender receives an NDR with the errors -

Your message wasn't delivered because the recipient's email provider rejected it.

Remote Server returned '550 5.7.520 Access denied, Your organization does not allow
external forwarding. Please contact your administrator for further assistance. AS(7555)'

This article explains that although the error initially points out the issue on the recipient's (Kerio Connect's) side, it needs to be resolved at the sender's end by fixing their external forwarding rules.

Information

• These errors usually occur when the email is being sent from Microsoft Office 365 accounts like outlook.com, hotmail.com, live.com, or msn.com. So, for the rest of the article, only these accounts are considered.
• The initial error message says that the recipient's email provider rejected it. Usually, the emails can go through multiple hops before they are received on the Kerio Connect server. So, the rejection can come at any intermediate servers.
• The sender's organization could have blocked the automatic forwarding of emails to external domains due to security reasons and in that case, the NDR with the Remote Server returned '550 5.7.520 Access denied, Your organization does not allow external forwarding. Please contact your administrator for further assistance. AS(7555)' error is received by the sender. This is also explained in detail in the article configuring automatic external email forwarding.
• The sender will need to work with their IT team to figure out their automatic spam filter policy. They can also use the configuring outbound spam filtering article as a reference.