Accidentally marked as spam - Now unable to undo!

Hi Sivaprasanth,

First things first, please check if you had made any settings in other device such as mobile phone or other PC, because if you are using an Office 365 account, Outlook will auto sync the emails with those devices.

To help you troubleshoot this issue, please open your Outlook client, in the Home tab > Junk > Junk E-mail Options > Safe Senders > Add > enter the email address > OK, and checked check boxes before Also trust e-mail from my Contacts and Automatically add people I e-mail to the Safe Sender List.

In addition, please also check if the address is in the Blocked Senders, if so, please remove it.

Moreover, if you have admin permission to sign in Office 365, you may create a mail rule to change this situation by followings steps (if you don't have admin permission, please contact admin to help you with it).

1.Sign in Exchange Admin Center > mail flow > rules > Create a new rule. 

2.Under Apply this rule if drop-down list, select The sender is, and then select the user that you want to receive email from.

3.Click more options, then under Do the following drop-down list, select modify the message properties > set the spam confidence level(SCL) > Bypass spam filtering.

4.Under Properties of this rule: checked the check box before Audit this rule with severity level and then select High in the drop-down list, then click Save. Below picture for your reference:

Hope above information helps. If you still have any problem with this issue, we need following information for analysis:

1.The detailed information of your Outlook client version. See: File > Office Account > About Outlook.

2.What type is your account? Exchange, POP or IMAP?

3.Does this issue occurred to the other senders or just the specific one?

4.A screenshot of the rule that you created.

5.Please check if this issue persists in Outlook Web App.

Looking forward to your reply.

Regards,

Jennifer