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The email service luxsci.com scrubs a lot of information from your headers, if you wish [1].

Another well-known email provider allows you to scrub the IP address, but it isn't clear whether they mean to publicize it. So Google the name of your favorite email provider along with "hide ip address", and see what you can find.

[1] https://luxsci.com/extranet/info/email-smtp-anon.html

Fastmail includes it by default, but will scrub it if using port 565 for SMTP.

Unsurprisingly, Protonmail strips the IP address.

Protonmail does not support SMTP at all, so I'm not sure what you think they would be stripping.

Both Protonmail and Gmail hide the IP if you're using the web interface.

ProtonMail does support SMTP via a bridge:

https://protonmail.com/bridge/

tho not clear if dannyw tested with ProtonMail IMAP/SMTP Bridge or with ProtonMail web.

mailbox.org appears to not include it