After nearly drowning in an alphabet soup of anti-spam acronyms whilst trying to explain to a customer why email forwarding doesn't work, Kelduum decided to let off steam on our blog. https://www.mythic-beasts.com/blog/2025/01/29/the-death-of-email-forwarding/
@beasts Hmm, that probably rules out me forwarding family member’s email to their gmail or similar mailboxes to save on my storage then!
@jmb for Gmail, pull via POP3/IMAP works well enough, although a bit more fiddly to setup. And it should empty the inbox each time, so only briefly using storage on your account.
That works but due to mails being pulled rather than pushed they can be delayed for some minutes.
When I set it up myself I had missed the necessary step for the IMAP server to reload the TLS certificate after renewal, so when the first certificate expired the pull stopped working. It turns out that is not particular visible in the Gmail UI. I had to actually go to the relevant section in the settings in order to even see that there was an error.
I also never figured out how to get Gmail to know whether an email it has pulled from IMAP has passed SPF validation or not.