This is most likely occurring due to the fact that constituents do not currently have the ability select the postal mail opt-in/out status when making a donation on a donation form. When a new constituent makes a donation on a donation form a record is created for that constituent. If that form does not have the Physical Communication Opt-in data element added to it, the record created as a result of the donation will automatically have a status of does not accept postal mail in Luminate Online. When this record syncs to The Raiser's Edge in the plugin it will show that Do Not Send Mail To This Address is marked.  To prevent this please follow the steps contained in the video below.