It is obvious for other forms (마세요, 마십시오) because of the ㄹ 받침 and 으, but 마 is 말 + 아, which means it should be 말아. However, I have never seen that (and in speech it would be heard as 마라 anyway). Does this mean 말다 is an exception?
말다 is irregular among irregular verbs.
- 말다 (to stop) ・ 살다 (to live) ・ 날다 (to fly)
- 말라/말아라 ・ 살아라 ・ 날아라
- 마오 ・ 사오 ・ 나오
- 마/말아 ・ 살아 ・ 날아
- Because 말다 happens to be only used in imperative voices; i.e. pharses like *~하지 말겠다, *~하지 만다 sound ungrammatical and awkward.
- Because 말다 can't stand alone but like an auxiliary verb, like
But you can also use 말아 & 말아라; colloquially 말아 & 말아라 have been used recently, and accepted as (interchangeable) standard in 2015.