Continued Email Problems

We continue to have issues with our emails.  Recipients are saying they never received emails we've sent.  This is even happening to people with email addresses we have emailed in the past with no issues.  This is beyond frustrating, trying to run a business, paying for an email service that is starting to be useless.  We have to resort to sending emails through a different (free) email we use.  I need answers and hopefully solutions.