Why do some messages have a red ”X“ icon in webmail?

We’re occasionally asked why some messages in webmail have a red X on the envelope icon, like this:

webmail red x icon

This happens when another program or device marks the message as “deleted”, but hasn't yet sent the command to finish the process and move it to the “Trash” mailbox (called “expunging” the message).

This has other effects beyond webmail. Some programs or devices, like the iPhone, won’t show these messages in the mailbox at all.

What causes this?

A common cause of this problem is a rule in the macOS Mail program that has an action of “Delete Message”.

To avoid this problem, you can change the rule action to “Move Message: to mailbox Trash”.