Reply emails not going to client

Hello-

I have been having issues with my email replies not going to my clients. They are simply not getting them, Sometimes I get a unable to deliver message and sometimes not. What's going on? and how can we fix this? Mostly its with gmail accounts, but is affecting others as well

Our email addresses are- orders@burientrophyonline.com and angie@burientrophyonline.com

Help would be appreciated