Feature request: DKIM signing

DKIM is an important method to protect your domain by making it harder for spammers/phishers to use your domain as the ‘From field’.

By combining DKIM and SPF with DMARC, domain owners can eliminate almost all spam email sent in their behalf.

If WP Mail SMTP had optional support for this, users of this plugin would only have to paste one DNS TXT record to their domain generated by the plugin. It would be really easy.

Are you planning on adding this feature? If yes, when? If no, what would be the challenges in adding DKIM support?