If certain checks deposited through a Lockbox service are being returned by the drawee due to “Missing Endorsement” which I gather is for the payee, aren’t these items guaranteed by the bank of first deposit?
Also, in Check 21 is there such an endorsement as a ‘Virtual Endorsement?”
All advise is greatly appreciated.

If certain checks deposited through a Lockbox service are being returned by the drawee due to “Missing Endorsement” which I gather is for the payee, aren’t these items guaranteed by the bank of first deposit?

Yes

Also, in Check 21 is there such an endorsement as a ‘Virtual Endorsement?”

§ 3-416. TRANSFER WARRANTIES.