If i send in my tax return by mail, before the dealine. But they receive it after. is it a problem?

It just has to be postmarked by the deadline.  It is always a good idea to use a service that will provide you with a receipt or tracking so you know the IRS received your return.

To send your tax return by mail and get a receipt to see that the IRS received it:

https://www.usps.com/taxes/welcome.htm