J-bam

Interesting. I was transferred a little over 2 weeks ago and email forwarding seemed to work fine. Sometime around this morning, it stopped working and senders are seeing the following error:
host mxa.mailgun.org[34.160.157.95] said: 550 5.7.1
    Relaying denied (in reply to RCPT TO command)
I've reported this to support, but haven't heard back yet. Looking at my DNS records, I see the same mailgun MX records as described by other posters

Have the same problem. When Google moved my domain to square space I lost my dmarc and spf is borked, all my aliases I setup before go into a blackhole now. I wish Google or square space would have told us this beforehand. This is an old domain for me which is no longer monetized, though I can imagine if this was my primary source of income it would have cost me hundreds of dollars in lost revenue, what were they thinking? Thanks for those who replied with the workaround. I'll deal with this after the memorial day holiday and hope I can repair the damage. Square space and Google domains , if anyone on staff is reading this, you really need to post a FAQ to get people back up. While email admin is a skill in my hat, I can imagine there's a lot of broken customers now with no idea how to make repairs.

Found a great fix for this. Ditching squarespace since google fobbed us onto them, and using cloudflare as my new nameserver, they have a completely free catchall for email forwarding.

Same here.  Customer service is useless. 

Hi,
My domain got transferred yesterday; but it wouldnt show in my squarespace domains.
Due to this I have now stopped receiving my emails to my hello@customdomain.com email. 
Thank goodness I have forwarding set on this email - so the forwarding still helps.
I received the migration notification 21 hours ago - anyone else having the same issue?

It worked, but when I send an email it fails on validation and is considered as a phishing

Authentication-Results: spf=pass (sender IP is XXXX)
 smtp.mailfrom=gmail.com; dkim=none (message not signed)
 header.d=none;dmarc=none action=none
 header.from=XXXX.dev;compauth=fail reason=001