Don't self-hosted images in the email signature allow you to track whether the email was opened or not and potentially where it was opened? I can only imagine they want you to pay for that privilege as a service.
replyGmail already proxies embedded images so you'd only be able to track when an email was opened the first time. Anyway, I have been sending (very low volume) of mails with self-hosted images embedded in the body for years without problems with Gmail but Microsoft doesn't like my VPS IP because sometimes there are temporarly bad actors on the same block. So I guess it's mostly random luck.
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