Why are some of our sent emails going to spam folders.
"This message was created automatically by mail delivery software. A message that you sent could not be delivered to one or more of its recipients. This is a permanent error. The following address(es)"
I'm sorry to hear you're having a problem with your mail going to spam. The most important part of that error message comes after the addresses are listed-- the reason a piece of mail was rejected should be provided by the error message.
The particular reason a message was rejected will determine the best advice for this scenario. If it was rejected by the server itself, it's possible that mail from your address is blacklisted, either by that server or by a more widely circulated blacklist. If it was blocked from leaving the server, it is possible that certain keywords and attachments were the issue.
Please start with our guide on the topic, and make sure you have your DMARC and SPF configured correctly.