Hi @Ashutosh Bhalerao ,
As far as I know, it is not possible to deploy conditional formatting via Group Policies.
You can add a rule from Exchange Admin Center which could apply disclaimer or modify the subject of every received message.
To apply disclaimer, please follow the steps in this link: https://learn.microsoft.com/en-us/exchange/security-and-compliance/mail-flow-rules/disclaimers-signatures-footers-or-headers?redirectedfrom=MSDN
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