Check Data Has Been Entered Before Submitting

i currently have an email button setup, so when the person has filled in the sheet and clicks the button it sends the active worksheet to the specified email address in the code

i need an extra step in this code to check that data has been filled in, in certain cells before the email is sent

for example i have numberous cells but in cell i17 if something is entered then there must be something entered in cell i19. If there is nothing filled in cell i17 then the data validation can ignore this. Basically it needs to check that corresponding cell(s) are filled in "if" the first cell is filled in (make sense?)

i would also like it to check certain cells are filled in (mandatory cells)

here is the email button code, is there anything i can implement in that to produce the above results? or if this cannot be achieved maybe a warning box to pop up before it sends to say "Please make sure everything is filled in corrently before submitting" (something like that)