If you are seeing a 503 MAIL FIRST error in a client's outbound queue, this could stem from a number of issues:

1: The MAILFROM field in the message is blank

2: If from a Distribution list, NDR tracking is not enabled.

3: If part of a multiple recipient message, one of the recipients addresses is blacklisted within Reflexion

To solve both 1 and 2, please open up the mail contact/user object properties in AD and confirm that the "Manager" checkbox on the Organization tab is selected, and filled in.

If this does not solve 2, please run the following script where "DistributionGroupName" is the name of the Group object:

Set-DistributionGroup "DistributionGroupName" - ReportToOriginatorEnabled   $true

To solve 3, please provide a list of the recipients to Reflexion Support and they will compare against our internal blacklist.

Please note, that causes 1 and 2 are the most likely reasons for this.  Please check these first.