Just got through an hourlong hold with Fidelity, where she had to speak with 5 different back office reps to find this information.
If you use Quicken Bill Pay, or any other 3rd party bill pay service, and Fidelity Cash Mgmt is your funding account, ***DO NOT*** use the 'Send a check' option. The biller will receive the check, and show it as "cashed", then like a week later you'll get a notice from the biller that Fidelity refused to the funds request.
It appears (again, took 5 different back office people to get this info) that Fidelity's policy is to blanket refuse any and all third party checks. So unless the physical check being cashed is from the checkbook that Fidelity themselves sent you, they will automatically reject it.
This policy isn't published anywhere that either I or the Fidelity rep could find.
Fidelity touts their deep partnership with Quicken + the fact that the Cash Mgmt accounts are 'just like a checking account'...yet no checking account I've ever had in 20 years of using QBP has had this policy.