Upon creation of my account, I provided my email, but did not get a message to confirm the email. Nonetheless, the following message was displayed at https://wikibase-registry.wmflabs.org/wiki/Special:ConfirmEmail :

A confirmation code has already been emailed to you; if you recently created your account, you may wish to wait a few minutes for it to arrive before trying to request a new code.

After clicking the "Mail a confirmation code" button, the following additional message appeared:

Wikibase Registry could not send your confirmation mail. Please check your email address for invalid characters.

Mailer returned: Unknown error in PHP's mail() function.