One can in fact send emails from Workers with their Cloudflare-registered domain name through MailChannels [0]. We use it to send MagicLinks [1]. And intend to impl DKIM signing, soon [2].
[0] https://blog.cloudflare.com/sending-email-from-workers-with-...
[1] https://github.com/celzero/otp/blob/f6bb5593c5173a2844178638...
[2] https://mailchannels.zendesk.com/hc/en-us/articles/712284923...