summertrain wrote: What would actually be the procedure, as has happened, if you were taken off ESA due to Royal Mail not delivering the ESA50 
If a claimants ESA has been stopped because they failed to return the ESA50 then they will be found Fit for Work due to a technical failure, this is an appealable Decision, so you would first have to submit a Mandatory Reconsideration request.
Legally the DWP and it's contractor ATOS can assume that any letter sent is received by the claimant, but they must show that it was correctly addressed and that the "letter" was actually produced, if they cannot do this and from previous experience the audit trail on the ATOS systems are not as good as those on the DWP ones, then it may be possible to get the Decision overturned.
From the claimants view they need to show that the correct address was available to the DWP, you do not need to prove it was available to ATOS as there is no requirement for a claimant to inform them of any changes. If there have been previous problems in delivering mail, from any source, to their address then they will need to get evidence of this or if there has been any reason why their mail might not have been delivered, for example; a strike affecting their area.
If the MR does not overturn the Decision then an appeal can be lodged, however, as this is a technical failure, ESA is not payable during the appeal. We normally recommend a claimants makes a new claim for ESA, whilst this is also not payable unless a new condition or deterioration can be shown, it will go into payment once six months have elapsed or a WCA is carried out.
Gordon