I'm not GP, but I've had transitions denied while attempting to purchase something because the retailer's code flagged my email address as suspicious because it contained their name in it.
The most shocking thing was that I was calling them regarding an issue in which they required me to prove my identity, and yet the person I spoke with didn't seem to be well versed in security measures.
Also: I use a separate alias for every company (and sometimes individual) I deal with. In the 25 or so years I've been doing this, so far I'm up to over 1,000 aliases.