Panasonic KX-FLM671 - Manual (Page 29)

3. Printer
29
Do not attempt to print on any of the following types
of envelopes (these may cause damage to the unit):
L Envelopes with clasps, snaps or tie strings
L Envelopes with transparent windows
L Envelopes that use encapsulated types of adhesive
that do not require moistening but rely on pressure to
seal them
Side seam
construction
Peel-off strip Multiple flaps
Twisted Curled Bent-edged
Waved Dog-eared Self adhesive
Clasp Transparent window