Email service providers are always working to fight spoofing and spam, but sometimes the good guys get caught in the crossfire. A good example is when sending an email from and to the same domain results in a delivery failure.
Let’s say you’re using a third party service, like HubSpot, and you want to use your company's domain name as the ‘from' address to send your email (e.g. firstname.lastname@example.org). You then send this email to your teammate on the same domain (e.g. email@example.com). Your company's mail server sees an email coming from a HubSpot server. While the email claims to be from your company's domain, your company's mail server knows it’s not from your company's server and rejects it, as it suspects spoofing.
Don’t worry; there are steps you can take to let your company's mail server know that it should expect emails coming from HubSpot. Check out this article for more information.