When this first broke I read an article from the Columbia SC paper that said they got the emails back in Dec from the mistress’s ex-husband.
They said the emails were anonymous, and that they didn't act on them at the time because they couldn't verify them.
There have been separate stories that the emails were from her ex or from another boyfriend, presumably miffed that he was not her chosen one. She has denied both those stories and claims she is fairly certain of the actual source but can't reveal it.
As for Jenny Sanford, she claims to have discovered a letter from her husband to Maria some months ago.