I prefer to do mail order shopping because it's convenient.

I found a letter in the mail slot.

I prefer mail-order shopping because it is convenient.

I always throw away junk mail without even opening it.

I sent the important documents by registered mail.

I sent the important documents via certified mail.

I received an email from the mailing list.

I found a letter in the mailbox.

I received a package in the mailbag.