Electronic Disclosure and Burden of Proof

Once a Plan Administrator has obtained consent to distribute documents electronically - where does the burden of proof lie?  Say the plan administrator emails to the address on file, but does not request a return receipt, but the email does not kick back as invalid, if the participant then claims to have not received the document, who has the burden?  I've been looking for cases, guidance, etc., but the only thing I've found is the Thomas case wherein the employer posted on a website, but did not inform the individuals that it was posted.

Any further information/guidance/cases, etc. is greatly appreciated!