[Users] Red "S" in message Status field

[Jeremy]

Hi

Claws v3.13.2 Ubuntu 14.04

I've just started seeing a white "S" in a red circle alongside some
messages in my message list window. Normally this has the read/unread

I don't recall seeing this before and I don't think I've changed any
settings. My first though was that this was a SPAM flag but I don't
have any anti-spam plugins activated.

Could someone please enlighten me as to what this feature is. Thanks

Jeremy