Email Issues

I am having some email issues. First my emails are being sent but are never received by the person I sent them to. Its been happening for awhile- but over the last month has progressively gotten worse. Also importing my emails into outlook- there are constant errors. I have tried 3editing my STMP/ etc but nothing seems to be helping! This is a business email account and is making me look unreliable and unprofessional to my customers. Please help.