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> Check headers in outgoing emails for an origin IP (this one gets way too many sites)

Are there any workarounds for this, other than running mail servers on a separate network and IP range?

Use Amazon SES to send your email. SES actually conceals your origin IP -- unlike other providers like Sendgrid, which include it.

Run your mail server on a different provider and configure it to strip the relay headers - some mail relay services may also work for this purpose.