Is it possible to send someone a Nightmares-reducing action while they’re already in the Mirror-Marches? It seems like, if it is it’d be very useful if somebody could send me one Fallen London .
You can not receive mail outside of London (which the Mirror Marches are). The last possible time to read menace reducing mail is when the menace reaches 8 and you have not clicked “ONWARDS” yet. In the best case, you have a mail sitting ready on that moment.