A few years ago Paul responded to a fan letter from a group of students in Scotland, who wanted him to write a message to their beloved janitor, who was retiring.

https://www.thescottishsun.co.uk/news/800798/paul-mccartney-letter-edinburgh-janitor-andy-cairns-retirement/

Paul responded, so I tend to  think his fan mail is read somehow.   It may be a case of someone screening his mail for him, and passing on the ones they think he should see.  It could also  be that the fan mail isn't thrown out, he just gets so much, and doesn't have the time to read or respond to it all, but every now and then he'll look at it.

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