Email is not working properly!!

First off, I am not receiving all of my emails right away (it'll take 24 hrs most of the time), sometimes not at all. My customers are not getting my emails not at all or again 24 hrs later. I am getting a lot of spam mail. Never use to before at all. What in the world is going on?!?! I need this fixed ASAP! I am losing customers and my business is not doing well at all because of the communication error! Please let me know what I can do to get this fixed, I do not want to set up a temp. email account.