Either would be correct. An is used when the following word starts with a vowel or a vowel sound, so if you read out the MBML starting with the em sound it would make an appropriate.
A also works because the actual letter that begins the next word, in this case an abbreviation, is a consonant. In this case it would be more definitive if you read the abbreviation out as the full title.
A good example of when this comes into play is the word hour. Even though it starts with a consonant, it is a silent one, and the word begins with a vowel sound. Meaning that in this case, an hour would make more sense grammatically.