>wouldn't that also block attachments the users send out on their own?

Depends on the settings. It can block certain types in both directions. It can also be configured using a transport rule on a hub transport or an edge transport and then it can also be configured on a firewall. That is why checking with their admins is a good idea as a first step. If you send an email separately via outlook with an attachment of the same type to someone inside their system is the attachment filtered? Or is the attachment only blocked when it is sent from your vfp app?