Hi @Garrett ,
What's the version of your Outlook client(File>Office account>About Outlook), please first ensure that you have updated to the latest version of Outlook.
Did this problem happen only recently, or did it happen since the first time you used the Outlook client? If it occurred only recently, have you performed any specific operations before it(such as install any add-ins or others)? In order to avoid the interference of add-ins, please try to restart your Outlook in safe mode to check if the issue has any difference.(Windows + R> type "outlook.exe /safe">enter)
If I send emails from Outlook, they go to the recipients' spam folder. If I send emails from Gmail, they go to the recipients' inbox.
In the above test, are you sending the same email to the same recipient on a same PC ? Could the issue be reproduced if you send a message in plain text from Outlook client? In order to further confirm your issue, if it's possible, it's suggested to add the recipient's account on your Outlook with a new Outlook profile and re-send a message to him for testing.
If it could work normally on Outlook client, the issue may be more related to Gmail client, due to here we mainly focus on general issue on Outlook desktop client, we may know little about the spam policy in Gmail server or client. If you are working with gmail server, I have researched a lot about it and found a related thread that mentioned many policies and settings about gmail, please kindly check if it could also be helpful to you.(Please Note: Since the web site is not hosted by Microsoft, the link may change without notice. Microsoft does not guarantee the accuracy of this information.)
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