HP 1100A - Manual (Page 158)

158 Appendix C Paper specifications EN
Envelopes with adhesive strips or flaps
Envelopes with a peel-off adhesive strip or with more than one flap
that folds over to seal must use adhesives compatible with the heat
and pressure in the printer: 392 °F (200 °C). The extra flaps and
strips might cause wrinkling, creasing, or jams.
Envelope storage
Proper storage of envelopes helps contribute to good print quality.
Envelopes should be stored flat. If air is trapped in an envelope,
creating an air bubble, the envelope may wrinkle during printing.