Can The Contents Of A 'PO Box' Be Checked For Mail By A Computer?

No I am afraid we have not reached this level of interface between snail mail and electronic mail. They would have to have a record of each post box holder e mail address and update it hourly, they can barely keep up with sorting and delivering mail.

However if this idea appeals to you, you could register with someone like mailboxes  pay there fee, and make friends with someone there who could alert you if mail came for you, with a small bribe of course. Yet, I think it would be better, if you dragged yourself away from your computer and wandered down to the postbox to check your mail,  exercise does you good.