You may receive error message “The property keyword isn’t supported” when run cmdlet below to delete mail content from user mailbox’s.
If you see in cmdlet in above screen, I have done search query based on Subject :”FW: Full BAckups Report.” which is an forwarded message by other mailbox. in Serach-mailbox cmdlet search scope is very restricitive and can be use with very care. In above cmdlet if i remove part “FW:” then it works. So you need to make sure valid search query if you see this error in search-mailbox.
Exchange Search keywords
| Property | Example | Search results |
| Attachments | attachment:annualreport.pptx | Messages that have an attachment named annualreport.pptx. The use of attachment:annualreport or attachment:annual* returns the same results as using the full name of the attachment. |
| Cc | cc:paul shencc:paulscc:pauls@contoso.com | Messages with Paul Shen in the Cc field. |
| From | from:bharat sunejafrom:bsunejafrom:bsuneja@contoso.com | Messages sent by Bharat Suneja. |
| Keywords in retention policy | retentionpolicy:business critical | Messages that have the Business Critical retention tag applied. |
| Date when messages expire according to policy | expires:4/1/2010 | Messages that expire on April 1, 2010. |
| Sent | sent:yesterday | All messages sent yesterday. |
| Subject | Subject:”patent filing” | All messages where the phrase “patent filing” appears in the Subject field. |
| To | to:”ben smith”to:bsmithto:besmith@contoso.com | Messages that have Ben Smith in the To field. |
You have to be assigned the following roles to search for and delete messages in users’ mailboxes which is notassisgn by default to any security role group:
- Mailbox Search This role allows you to search for messages across multiple mailboxes in your organisation. Administrators aren’t assigned this role by default. To search multiple mailboxes, add yourself as a member of the Discovery Management role group.
- Mailbox Import Export This role allows you to delete messages from a user’s mailbox. This role isn’t assigned to administrators by default. To delete messages from users’ mailboxes, add the Mailbox Import Export role to the Organisation Management role group.
All about Microsoft 365 & Teams 
Exchange 2010 SP2 This worked no errors:
Use any identifer to get the mailbox group you want: Get-Mailbox -resultsize unlimited or -idenity or whatever..,
get-mailbox -OrganizationalUnit “Cheese.dom/SpecialAccess/Awsome/Users” | Search-Mailbox -SearchQuery subject:’Party in the Office’,from:’Bob@Cheese.dom’ -DeleteContent
Good luck
Yes, I came to know later :). My test was with Exchange 2010 SP1. Thanks for sharing !!!!
Do not know why but property keywords are localized: for example in a localized IT-it system -SearchQuery subject:xxx became -SearchQuery oggetto:xxx