Servers from self-hosted accounts tend to be more restrictive than others, at least from my personal experience. Here is my suggestion: # - click file>offline>download for the first account. This is to ensure that you have 100% of the headers and content downloaded. # - now, COPY the inbox to Local folders, either by exiting thunderbird and copying with windows file explorer or by using the importexporttools addon (highlight Local Folders, rightclick and select the import mbox option and then locate and select the other account's inbox file. This copies the messages to your local folders. # - now, create a manually-run filter to upload to the other account. My intent in this suggestion is to minimize activity that may be overloading the hostinger server.