from PUB 590-b
Deemed IRAs. A qualified employer plan (your 457-b plan) can maintain a separate account or annuity under the plan (a deemed IRA) to receive voluntary employee contributions. If the separate account or annuity otherwise meets the requirements of an IRA, it will be subject only to IRA rules.
you must have this separate account maintained under your employer's plan and make the QCD from it not the 457-b portion. Generally, the rules for a 457-b are not consistent with the rules for an IRA that's why the separate "IRA" account is required.
