BleepingComputer.com: Not Receiving Emails on Mac

Hi. I use a Macintosh computer and use their default email program. My email address runs off the homepage website of my business. Recently, some emails that clients are sending will not come in the first few times that they try emailing me. I have even received forwarded emails that were previously addressed to me. Calling clients to ask them to resend their information has caused some frustration, and there is a possibility of losing them because of this issue. I look through my mail, junk, and trash, and the emails are not found.

If there is a way to fix this problem, whether it be a simple change in settings, purchasing new email software, or even calling up a computer expert to examine the mail system, please let me know. If there is no way to fix this, could someone at least please explain why this might happen so that I can explain to clients why their emails might not have been received?

Thank You.