While trying to applicate Signature for Google tenants, you may have to deal with the following error message:
“Set signature error for firstname.lastname@example.org: Google.Apis.Requests.RequestError Signature exceeds maximum length of 10000 characters  Errors [ Message[Signature exceeds maximum length of 10000 characters] Location[ – ] Reason[invalidArgument] Domain[global] ]”
It is a Google limitation that can’t applicate a signature with over 10000 characters.
You have to reduce your signature characters’ volume.
You can edit your signature template here.