We have KM 6136 which is the newer and nearly the same as the 1250, along with a KM C7090 and C6085. We've printed peel & seal a few times without any issues, but we use the envelope fuser. If I recall, there may have been some slight wrinkling along the top towards the tail, but that even happens with regular gummed envelopes. We've always been able to ask our paper supplier for samples before doing a large order. I'd suggest you try that because I don't know how the older C7000 will perform...but probably similar to other gummed envelopes if you've been running them.