This issue started about a month ago, I was communicating with a client back and forth through email, then all of a sudden, they weren't getting my responses or new email communications. They said I'm not in their spam folder, so I used a gmail account to complete my work with them.

Then as of last week, I had been communicating with a client via email, and all of a sudden, she's not getting my responses.  I ended up calling today to make sure this was the issue, and it was indeed.  I've asked her to check her spam folder but hadn't heard back yet.

It seems to be a problem with our domain, not just a specific email address because they also weren't getting their invoices that also come from a different email account using the same domain.