Important Note: In January of 2020, Mailgun removed the ability to receive replies from their free tier. Those who created MailGun accounts prior to the change should be grandfathered in as far as being able to receive mail, but if you created your Mailgun account after December of 2019, you'll need to upgrade to Mailgun's lowest paid plan in order to receive emails in HighLevel. Please watch our How To Set Up Replies With Mailgun video for more information.
There are two different ways to set up Mailgun
1. One Subdomain (preferred option)
You can set up one sub-domain in Mailgun for your marketing agency domain and use that same sub-domain for email sending across all of your client accounts in HighLevel
Pros:
Easy setup - only need to setup one subdomain for your domain which you already have access to
Emails are white labeled to your marketing agency domain
Cons:
If your domain gets disabled (gets a bad score due to bounce rates) email will stop working for all of your clients since all of their accounts are powered by the same domain. The solution: don't send spammy emails to poor email addresses :-)
2. Multiple Subdomains
You can set up a subdomain in Mailgun for each of your client's domains and use that subdomain for sending for their specific HighLevel account
Pros:
Emails are white labeled per the client domain
If a client's domain gets disabled email only goes down for their client account
Cons:
Difficult setup - time consuming having to setup a subdomain for all of your clients and getting access to each of their domains