I would also like to use SendGrid for a multisite/network with 100+ sites.
I guess things to consider are the following.
- I believe emails are currently sent from the base/network site, not the sub-sites (the header info seems to support this)
- I use Domain Mapping, so each sub-site has its own custom domain name.
- It would be good to manage settings once under the main site or via wp-config.php – as adding settings to each sub-site would be time consuming.
- White-listing the main domain would also be easier than having to white-list all sub-sites.
- I use Contact Form 7 and use Additional Headers for Bcc and Reply-To, so not sure how SendGrid would work with that?
Let me know if you have any questions, I am happy to test and give feedback if required.
David