msg:binding_group — Sets the binding_group to the named binding, if one is provided.
Sets the binding to one in the named binding_group, if one is provided, using logic similar to the Sieve function
ec_set_binding_group. Returns the current or new value for the binding_group.
If the adaptive module is enabled the logic for choosing a binding is as follows:
An unsuspended binding local to the MTA
A non-local unsuspended binding
If all bindings are suspended, a local binding is randomly chosen
Enable this function with the statement