Hello 
After receiving an email from Mailgun about their upcoming price increase — doubling the Flex plan rate from $1.00 to $2.00 per 1,000 emails — I decided to switch over to Emailit today instead.
A quick note on setup
Overall, the setup was completely smooth. The only issue I ran into was the one discussed in this topic:
530 From header does not match MAIL FROM address
The fix was simple: the “Reply by email address” must not include a prefix like
replies+%{reply_key}@mydomain.com.
That pattern caused the error above. I removed the replies+ prefix, and everything started working perfectly.
My working configuration
After adding the DNS records and creating an API key in Emailit:
app.yml
DISCOURSE_SMTP_ADDRESS: smtp.emailit.com
DISCOURSE_SMTP_PORT: 587
DISCOURSE_SMTP_USER_NAME: emailit
DISCOURSE_SMTP_PASSWORD: "api-key" # (obtained from Emailit)
#DISCOURSE_SMTP_ENABLE_START_TLS: true # (optional, default true)
DISCOURSE_SMTP_DOMAIN: mydomain.com
DISCOURSE_NOTIFICATION_EMAIL: notifications@mydomain.com
Site Setting:
Reply by email address: %{reply_key}@mydomain.com