A MULE email account means different things.

It may refer to an address maintained by an individual for the sole purpose of signing up for online offers or registering with a websites which require an email address. MULE accounts are used in order to prevent the loss of privacy and to avoid spam.

Alternatively it could refer to an account used to commit fraud. In this scenario the email address is referred to by a spam campaign. The spam campaign will usually offer the chance to be an intermediary for transactions involving large sums of money, that just so happen to require someone in the intermediaries position, who is offered a share of the large transaction for their troubles.