It is a possibility. However, at least 3 of the 8 Democratic primaries outside of the margin of error don’t allow early voting by mail (Alabama, Mississippi, and New York): http://www.ncsl.org/research/elections-and-campaigns/absentee-and-early-voting.aspx
If the mail in voting did skew exit polls, the exit polls unaffected by it are enough evidence to warrant investigation into fraud.
Also, if mail in voting affected exit polls, they would reasonably skew the Republican side too, since Donald Trump is handily winning elderly voters, but the first exit polls have been spot on. It would be worth it to do more research as well though.