What does "writeline" return? Where are you using writeline from? Is it from a command prompt? Where are the addresses stored? Is there another way to get them?
In any event, when you call whatever you are calling to get the addresses, if you are getting one at a time, you need to concatenate the output like this:
sMailTo = sMailTo & ";" & writeline
Then you will need to remove the last ; from the line after you get all the addresses. Maybe
sMailTo = Left(sMailTo, (Len(sMailTo) - 1))