Courtesy forwards have been a standard feature of e-mail systems about as long as there have been e-mail systems. A user moves or changes jobs or something, and rather than just closing the account, the mail system forwards all the mail to the user's new address. Or a user with multiple addresses forwards them all to one place to be able to read all the mail together. Since forwarding is very cheap, it's quite common for forwards to persist for many years.
Unfortunately, forwarding is yet another thing that spam has screwed up. If you just forward all the mail that arrives at a typical address, most of what you'll be forwarding is spam. From the point of view of the system you're forwarding to, you're the one sending the spam, and they're likely to block you.
Fortunately, there are some ways to mitigate the damage.
If you do these, you should get mail through to most places. The local pickup trick will get mail through to anywhere.
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