Outbound emails not working

I've read the forums on previous issues, but I don't understand how to check on why our domain emails aren't being received. It appears we always receive emails, but sometimes our outbound are not received. We never send mass emails, although we do send one-off emails that are oftentimes not responded to by the recipient. Example: customer asks for a quote, we send a response, then they call to confirm an installation or don't respond at all. We are a small business with 4 email addresses. What can I do to make sure our emails are received? Thanks!