SMH is an acronym that stand for "Signal Message Handling." SMH can be used when someone has sent an email and would like conformation that the email has been received.
SMH can also stand for "shaking my head." In this sense SMH can be used in the place of words such as disappointed or confused. SMH is the action that sometimes accompanies the feeling of disappointment or confusion.
Sally: Hey, I sent you the email about the report, SMH.
George: Ok, I'll check for it when I get home.