Earn credits for referals and receiving mail (I would advise creating a new email address for this one!). Redeem credits for vouchers, cheque or paypal credit. They ask that you refer UK residents where possible, but don’t specify that you have to be UK resident to sign up.
MailPay can help you earn money and save money. Build a member profile and ‘market’ yourself to companies who pay to email you. Each email you’re sent earns you 25% of what was paid to sent it, up to 26p per email.
Invite others to become your buddies and you’ll earn 25% when they get sent emails too.
Peoplestring has a similar concept, but they are based in the USA. Anyone can sign up but the mailbox option is only available to US residents. You do get paid for referalls though, so make sure you target the US market if choosing that route.