Yes you can build a house outside of the towns in the free areas
Spotlight #7 (https://fracturedmmo.com/feature-spotlight-7-towns-governors-politics/) and the Q&A after it would probably answer most of your questions with what we know to this point. (https://fracturedmmo.com/july-23-24-double-qa-livestream/)
Needing to upkeep the mine as part of the city like the other buildings/structures is an interesting idea. I wonder if you would be able to then tax the characters mining to support the upkeep to keep the mine safe and stable. Something like x% of your total mined ore goes to the town's treasury.
I think having a mail system to contact another player would be interesting but due to what we know currently about the travel I don't think it should be used to store or send items.
In WOW it made sense since there was an auction house you could drop your goods off at to sell and if someone purchased it would get delivered that way. The way Fractured seems to be set up is there will not be a global/race auction house. Each town will have sellers. So if the best blacksmith is 4 towns over I need to take the risk of traveling with the money to go get whatever I'm buying and then travel back. Or the blacksmith needs to take the risk of traveling to another town to sell their goods.
A player/guild run FedEx or UPS system would be interesting. Especially if you can do insurance on packages/goods just in case 
today is all we have, yesterday is gone and tomorrow never arrives.
Whoohoo! Congrats everyone