It could be that the virtual keyboard on Android appends a space character to the email address (which is not a valid character in email addresses).
Of course this blank should not be part of the email address - the keyboard just appends a space after each word you are typing in. Nobody would expect that it would be possible to submit email Addresses which contain a space at the end. So, we safely can discard space characters.
This is implemented since mpform 1.3.39 (November last year). So, maybe updating mpform helps for you, too?