"Let's design better emails for climate" is not a great take. For starters, consider not sending those promotional mails at all, because likely most of your emissions are associated with the product you're selling; and yeah, capitalism is the ultimate issue here. (Also, if you're putting third party trackers into your mails I hate you anyway.)
And then if we're strictly looking at IT... So much energy is pissed away for video streams, huge databases, blockchains, and AI. Those semi-legitimate marketing mails probably don't even register next to mails that are strictly spam.