Hi, we've been seeing problems like this with Mailgun for about a year (with the support ticket record to show for it) and I'd love to get in touch. FYI your email address is not currently visible in your profile (should be in the 'about' section).
Is this per-project, or per-customer? I run https://spa.mnesty.com/ with you, which has a 1.5% bounce rate (I'm guessing that's high? I don't really know), and I wouldn't want it to be interfering with my personal email (0% bounce) that I also run with you.