On the administration level it is possible to (in)activate the authentication of the sender for e-mail upload.
This setting can be found through Settings (Cogwheel) > Administrations > Pencil > Settings.
This option is relevant when e-mails from suppliers, automatically forwarded, are not being processed. The Sender Policy Framework (SPF) settings from the supplier are responsible for this error. Disabling the sender authentication will make sure all next e-mails will be processed.
The Mailbox (Settings (Cogwheel) > Mailbox) shows all unprocessed e-mails regarding the SPF-error. Click in the Mailbox on the Magnifying glass and see if the error SPF check failed is shown.