In the current implementation we assume that ucan/attest is issued by the principal doing validation. And because ucan/attest is tied to specific delegation it introduces unnecessary loops where agent that did:mailto delegated capabilities needs ask re-request service to attest delegations every time did:mailto gets delegation even if it is covered by first attestation.

We can avoid this complication if such agent could arrange delegations on it's own as long as it has been delegated ucan/attest capability by the did:mailto principal.