Are you using Digital Ocean IPs for your mail server? I can see from here that your mail server is using Digital Ocean... I might be wrong. Anyway, I suggest you use Windows Live Domains for your mail server and for LoE outgoing such as for your forums' SMTP, you should use SendGrid.net, which costs a few cents per thousand emails or so. If you need help getting it set up, PM me.
Sorry for double post. Anyway, if you try the above configuration out, it's problem solved. I use WLD + SendGrid and it's a great combo. SendGrid accepts applications very quickly and they allow you to create multiple user credentials per account in the event that you have multiple applications using SMTP.
Oh - and if you're using phpmail(), then I suggest you consider SendGrid ASAP for both security and email receiving reasons.