Microsoft deliverability

Hey Lori-Merchan! Thanks for the tag, MaggieBrennan!

While I certainly can't purport to know more than a person who actually works at Microsoft (omg hi DavidO! How cool!), I would start by digging into your spam complaints. Complaints are the people who got your email in the inbox and clicked the "spam" button, so they're extremely damaging to your reputation but also very revealing! I like to perform spam complaint audits in which I put myself in the recipients' shoes and try to think about why they may have reported the mail as spam. Some common reasons are:

• The recipient did not sign up. Maybe someone else entered their email address accidentally or on purpose. Perhaps your signup form is collecting invalid addresses and needs to be secured with a captcha and/or honeypot.  Maybe they did give you their address but did not want emails from you, and there was no way to opt out of messaging prior to receiving some. Asking people if they want your emails, allowing them to say no and still interact with your app/site/product, and double opt-in will solve the vast majority of deliverability problems!
• The branding/content is different from what would be expected based on the signup method. Is your "from" name correct and obviously you? Is the subject line relevant to the recipient? Is the entire look/feel of the message consistent and easily recognizable?
• The unsubscribe link is missing, hard to find, or broken. People will report mail as spam when they can't opt out because it's the only option and it's available 100% of the time.

You can segment out a portion of complaint reporters and review their engagement history while taking the signup method and list hygiene processes into consideration. If someone is reporting mail as spam quickly after signing up, then it sounds like they're not really signing up, they're just giving an email address because it's required. If people engage for a while but eventually report mail as spam, then it may be difficult to opt out or the opt-out mechanism is unclear in some way. It is also possible that the content or cadence has changed, resulting in mail that is no longer relevant, or sent too often. If you perform a similar exercise with all types of outcomes (bounces, unsubscribes, conversions), you should get a better idea of what's working and what's not, or what types of recipients respond best to which style/frequency of messaging.

Hope that helps!