If the e-mail address of the current administrative contact for the domain is invalid, registrant won't receive the confirmation e-mail and won't be able to confirm the transfer.

The losing registrar can deny a transfer for several reasons:

Evidence of fraud;

UDRP Action

Count order by court of competent jurisdiction

Reasonable dispute over the identity of the Registrant or Administrative Contact

No payment for previous registration period (including credit card charge-backs) if the domain name is past its expiration date or for previous or current registration periods if the domain name has not yet expired.

Express written objection to the transfer from the Transfer contact.

A domain name was already in “Lock” status.

A domain name is in the first 60 days of an initial registration period.

A domain name is within 60 days after being transferred.