email address doesn't get testing invitation in Fabric Beta

Question

I use Fabric Beta. I release it to 10 users, all @gmail.com created at the same time.
Unfortunately, one email address stopped getting invitations at some point.

Answer 1

As per Mike Bonnell's comment:

Probably best to email support(at)fabric(dot)io as we need the tester's email address to look at our mail server logs.

In general, this happens either when our mail server receives multiple bounces for the receiver's email address or they've unsubscribed from all Beta emails.

I did exactly that and within a few hours they had replied with enough useful information to get me back up and running.

(community wiki answer as this question ranks highly in google search for the issue I had)