When an email is sent from a shared mailbox, a signature is not added to the received message.
This occurs in hybrid environments where a local user account's mailbox has been converted to a shared mailbox using the Office 365 admin center, and – having shared the mailbox and removed the Office 365 license – their Active Directory account is then disabled.
To generate signatures, Exclaimer Cloud aggregates data from the Active Directory and from Office 365 however, data for disabled users is not aggregated. Additionally, disabled users cannot be selected when choosing who signatures should be applied to, so no signature is applied.
Note: User accounts in the Active Directory for shared mailboxes should not be disabled or deleted - this can lead to the deletion of both the shared mailbox and the user from Office 365.
Enable the user account in Active Directory (enabling the account does not require a license and it does not cause a security risk).
Once enabled and synchronised with Azure - and after your Azure data has subsequently been synchronised to Exclaimer Cloud - a signature will be applied to emails sent from the shared mailbox (according to your signature settings in Exclaimer Cloud).
Further details about creating and converting shared mailboxes in Office 365 can be found here.
Alternatively, you can create a brand new shared mailbox in Office 365, rather than converting an existing Active Directory user's mailbox. Shared mailboxes created in Office 365 do not generate a user account in the Active Directory, and do not require a license in Office 365.