Email messages cannot be Moved for Forwarded to another Account based on a Rule

🔍 The Scenario:

You're using Mail.app on a MacBook or iMac, and you've set up a rule that automatically moves or forwards incoming emails based on specific conditions (e.g., from one account to another, or into a folder/mailbox in a different account).

💡 What Happens Behind the Scenes:

1. Rules in the Mail.app (configured on your Mac) are executed locally — that is, the Mac processes incoming messages and applies your rules only while the Mail app is open and actively checking mail.
2. When the rule instructs Mail to move or forward an email to a different email account (e.g., from your iCloud inbox to a Gmail folder), Mail must connect to both accounts to execute the rule.
3. If the destination account (where the message is supposed to be moved or forwarded to) is:
   1. Not connected,
   2. Not authenticated (e.g., you got signed out),
   3. Offline, or
   4. Disabled in Mail settings,
4. Then Mail cannot complete the rule.

⚠️ The Outcome:

• You may see a message such as:
• “The message could not be moved to the mailbox ‘[folder/account name]’.”
• The rule fails, and depending on your setup:
  • The message remains in the inbox.
  • Or it may be partially processed (e.g., marked as read, but not moved).

✅ To Prevent This:

• Ensure all accounts involved in the rule are active and online in Mail.
• If the rule is critical, consider:
  • Setting up server-side rules (e.g., on iCloud, Gmail, or Exchange) that run whether your Mac is open or not.
  • Or using Mail extensions or automation apps (like Mail Act-On or AppleScript) for advanced workflows.

🧠 In Summary:

Yes, on a Mac, email rules that depend on moving or forwarding messages to another account may fail if that destination account is not active when the message arrives. This is because Mail.app relies on both accounts being live to execute the rule successfully.