The shared IPs, and even dedicated IPs, offered by both Mailgun and LC Email (a.k.a. Mailgun repackaged) are on Microsoft blocklists.

Currently, our only alternatives to these providers are using SMTP to send emails or switching out of GHL altogether. Yeah, email deliverability, something which GHL lacks, is that important, and has been the constant thorn in myself and my clients' side ever since making the switch to GHL.

The problem with SMTP is that it means absolutely NO granular email metrics (GHL claims that open and click rates still work when using SMTPs, but based on multiple tests and personal/client experience, the data passed back to GHL via SMTP is highly inaccurate).

GHL is such a good platform otherwise and this simple fix—adding API key from other SMTP providers, other than Mailgun—would entirely fix this issue.