One of my discoure install has particular majority of users using Microsoft outlook/hotmail email accounts and the microsoft spam rejection system does some weird filtering and puts almost every email sent via relay smtp (e.g. services like mailgun, elasticemail etc.) This had been creating a lot of trouble lately. I’ve spent days talking to mailgun support and after all the logs and tests, their final word was to try sending mails via their API instead of SMTP credentials.
I believe this may be an issue with the server’s originating IP. When sending via SMTP, the server that creates the message has its IP added as the “originating IP” into the headers. If the IP is not properly configured for SMTP, the recipient’s filters may flag the messages as Spam. I would recommend sending via our API instead. Messages sent through our API will use the sending IP assigned by Mailgun as the Originating IP, instead of the IP for your server.
as most reputable email services support API, can discourse support emails via their API instead of SMTP?