This is obviously normal if it’s doing its job properly but sometimes good emails get marked as spam.
There are a few potential reasons why this would be happening:
1. There’s something in their email that’s causing your email program or the email account’s server to mark it as spam – Usually this is done by either a keyword that’s triggered it or a remote image that’s embedded in the email body. Sometimes it’s an attachment that’s causing it, sometimes it’s even something in their signature. You should ask them to think about something that’s changed since the last time they were able to successfully land an email in your inbox.
Solution: They should send you an email with no signature, no attachments and just a short simple text only sentence and short subject line. Additionally, you can have another person in the company (using the same domain email as theirs) try sending you an email and if you receive it then you have successfully confirmed that it’s something isolated to that particular email address. Proceed to the 2nd possibility.
2. Email is blocked by your Email program – Their email account is blocked by you due to you manually labeling that email address as spam. This can happen by accident but it does happen!
Solution: You can fix this by looking in your email program to see if you have blocked their email somehow. Either way, if you found the block and tested again and it’s still not working or it wasn’t blocked by any of your rules or spam blockers, proceed to the 3rd possibility.
3. Your Domain Name is Blacklisted – Their email server is blocked due to someone in the company either having an email that was hacked and used by a spammer to send out mass spam emails or they themselves sent out a bunch of emails and got blacklisted due to a threshold being reached.
Solution: Lookup their domain name on MXtoolbox.com in the drop-down choose “blacklist check” then type in their domain name and click the Blacklist Check button. If every item in that list is “OK” or “Timed Out” then they’re fine but if you see a red item, that means their email server is blocked due to spamming or something else. They should contact their Email service provider immediately. Also, recommend they take a look at this article as well. Your Email Account has possibly been hacked