singed pdf files get blocked when user is sending from shared mailbox

1. To check mail flow rules: Open Exchange Admin Center (EAC).
Navigate to Mail Flow > Rules.
Check if there are rules to block or flag messages containing signed PDFs. If so, adjust the rule or add an exception.
2. Check Send Permissions for Shared Mailboxes: Ensure that users have been granted Send As or Send On behalf permissions for shared mailboxes.
Run the following command in the Exchange Management Shell (EMS) to check the permissions:
powershell
Get-RecipientPermission -Identity email address removed for privacy reasons
If you need to add permissions, run the following command:
powershell
Add-RecipientPermission -Identity email address removed for privacy reasons -Trustee email address removed for privacy reasons -AccessRights SendAs
3. Check the attachment filtering policy: Open Exchange Admin Center (EAC).
Navigate to Protection > Attachment Filtering.
Check if there is a policy to block PDF files or signature files. If so, adjust the policy or add an exception.
4. Check email security gateway settings: If you are using a third-party email security gateway (Proofpoint, Mimecast, etc.), check if its settings block signed PDF files.
Contact the support team of the email security gateway to adjust the relevant settings.
5. Check Exchange Online Protection (EOP) settings: If you are using Exchange Online, check the policy settings of EOP.
Open the Microsoft 365 Security and Compliance Center and navigate to Threat Management > Policies > Anti-Malware.
Check if there is a policy blocking signed PDF files, and adjust the policy if necessary. 6.
6. Check the digital signature of PDF files: Ensure that the digital signature of PDF files is valid and has not been tampered with.
Use a PDF reader (Adobe Acrobat Reader) to verify the validity of the signature.
7. Try to re-sign the PDF file: If there are problems with the signature of the PDF file, try to re-sign the file.
Use a reliable digital signature tool (Adobe Acrobat) to re-sign the PDF file.