Microsoft have responded to our inquiry. It appears that the recent influx of WebFaction customers 🎉 has caused a sudden increase our outgoing mail volume and that this is what triggered the block.
We've verified via Microsoft's own postmaster tools that there have been zero complaints regarding mail from our US SMTP and have provided this data to Microsoft for consideration.
We hope to follow up with a resolution to this issue as soon as possible. Thank you for your patience.
