What are email guests?
This use case allows you to invite someone (e.g. client, contractor) to a channel even though they don’t use Slack. This way, any message posted in this channel will be sent to all of the channel's recipients, whether they are Slack members or email guests.
Example uses include an agency setting up a #proj-acme-website channel with the internal team as Slack members and the email-reliant clients as email guests. MailClark creates an email address in the form of email@example.com, or firstname.lastname@example.org if you set up a custom domain.