Grammar

Hello,

I would suggest something like this:

The drawings should arrive by next Monday. Please email me if you do not receive them.

'The drawings are expected to reach your office by next Monday. If you have not received them by the expected day, please send me an email'.

The first sentence is fine. I removed "hence" from the second sentence because it is almost never used any longer, except in formal writing. I don't understand why you wrote "before the expected day," it would seem to me that if they arrive on time, that's good enough. Someone, years ago, might have said "drop me a line," which would mean to send someone a letter. I don't think you would want to use that with an email, however.

Setanta,
it is because I want to use 'before' to replace the 'by'. I thought their meanings were the same. Thank you theParser and Sentana

Thank you theParser and Sentana

You are very welcome.

By a certain date means on or before that date.

It all depends upon the register/the level of formality needed for the situation, LJ.