Hello Anne-Claire CHANVIN, thanks for your question! You can use @gpt4 and ask "Reply to the email saying X". You can attach the content by clicking the following button (see screenshot)
I recommend creating a simple assistant in Dust ex called @EmailReply with the custom instruction similar to "Your task is to reply to emails, use a friendly tone of voice, keep it short, etc". This way you can simply call your assistant each time without the need to add new content. It also allows you to plug it to custom datasource with the "seach" tool.