I used to have delivery problems (throttling errors caused by another customer) sending messages to Microsoft related domains (hotmail, msn, live, etc.) when using MailGun.
There was no progress at all when discussing this topic with MailGun's support so I created another account at PostMark [1] to choose provider depending on recipient's domain. No more problems when sending messages to Microsoft related domains.
Anyway, "silently discarded" sounds like you need to put in place DMARC reports [2] to troubleshoot what is going on there.
PS: I chose PostMark because I like its online documentation. Currently a happy customer, not affiliate at all.
There was no progress at all when discussing this topic with MailGun's support so I created another account at PostMark [1] to choose provider depending on recipient's domain. No more problems when sending messages to Microsoft related domains.
Anyway, "silently discarded" sounds like you need to put in place DMARC reports [2] to troubleshoot what is going on there.
PS: I chose PostMark because I like its online documentation. Currently a happy customer, not affiliate at all.
[1] https://postmarkapp.com/ [2] https://dmarc.postmarkapp.com/faq/