Stalwart before 0.16 does not know urn:stalwart:jmap, and rejects the whole
request rather than the one call when `using` names a capability it cannot
parse. The probe is only sent when the session advertises that capability, so
this should not arise — but if it ever does, throwing turns a server we can
still manage credentials on into a Security page that only shows an error.
Fall through to the endpoint those servers do have.
jcoffey-dev
c8fe822586