Very interesting. Was this an alias at a Fastmail domain, or a personal (virtual) domain?
As others have pointed out,
dictionary attacks by spammers are common. I think that Fastmail has some ways of detecting such attempts. When I had my personal domain at Fastmail accepting any alias using a wildcard several years ago (*@mydomain), I was getting a huge number of obviously dictionary attack messages, so I disabled the wildcard. Now when I try accepting any alias I get very few additional spam messages, so I think that Fastmail has tuned things up to prevent many of the high rate automated attacks.
But you can still get random dictionary spam messages. If the spammer sends from a respectable server at a low enough rate, there isn't much that Fastmail can do to block such messages. And they can use a list of usernames found at other domains for a wide range of other domains, and Latin usernames would be good ones to try. It's possible that a spammer was sending two messages a week to that alias, and when you enabled it you now see those messages.
Bill