I have "send mail" enabled in the general settings, and the recipient named in the advanced settings, but the bot never makes a move to mail at all. Eventually it gets stuck at a vendor with a full bag of items that I have checked to "not vendor" because it hasn't mailed them away.

Have I missed a setting to enable sending mail to alts?

Hello, check if you have mailbox in npc db (tab tools or in your profile)

Thank you. Does it add a specific mailbox or simply the one closest too me? If so, do i have to add every single one I wish to use?

the one closest to your profile (if WRobot can make good path to go to)