Hi,

It clearly was spam.

Okay, but that was not my question. If a spam robot insert a new posting to the forum and if you subscribed this thread, too, you get an email from the forum software. This notification message/mail contains the content of the (spam) posting. Again: Do you subscribe threads of the forum?

Has there been security improvements to prevent email injection since 2.3.3?

Take a look to the change log.

/Micha