According to one embodiment, in response to a request received from a messaging system to categorize a message received from a remote sender, a messaging extension associated with the messaging client is identified. The message without a recipient identifier (ID) identifying a recipient of the message is transmitted to the messaging extension via a first application programming interface (API). The messaging extension is executed within a first sandboxed environment and the messaging extension is to perform a content analysis on the message to categorize the message. In response to a first analysis result received from the messaging extension, a response representing the first analysis result is transmitted to the messaging system, wherein the response indicates whether the message should be delivered or filtered.