Add guidance on using appropriate sender address

Author: marckohlbrugge

skills/fastmail.md

@@ -18,6 +18,8 @@ User request: $ARGUMENTS
 
 **Always prefer creating drafts over sending emails.** Unless the user explicitly says "send this email", create a draft instead. This lets the user review before sending. If you're unsure whether to send or draft, ask.
 
+**Use the appropriate sender address.** When replying to a thread, check which address the original email was sent to and use `--from` to reply from the same address. This maintains consistency in the conversation.
+
 **When triaging multiple emails**, use AskUserQuestion to let the user decide on each email interactively. Suggest options like:
 - Archive (recommended for newsletters, notifications)
 - Reply (show draft preview)