LZ1 compression for incoming SMTP Mail

Does LZ1 compression not work for incoming SMTP-Mail?

I sent an e-mail with SMTP to my Domino server. LZ1 compression for the mail.box and my mail-file is on. But the Attachment was not compressed!(checked the size in document properties)

When I send a Mail with an attachment from my Notes Client with compression checked in the file-open-dialog, the attachment is compressed after sending.