No external content, as that can be used for open-tracking, and is susceptible to breaking. Rewrite the image url using a content-id, `cid:` https://tools.ietf.org/html/rfc2392
No external content, as that can be used for open-tracking, and is susceptible to breaking.
Rewrite the image url using a content-id,
cid:https://tools.ietf.org/html/rfc2392