Vicente Fox — the former president of Mexico — is not a big fan of Donald Trump. In the last few weeks, Fox has referred to the Republican presidential nominee as a "piñata" who is "empty in his head" and a "false prophet" who makes lofty promises that can't be fulfilled.

Now, as Trump and his Democratic rival Hillary Clinton gear up for the first presidential debate before the November election, Fox has once again lashed out at the New York billionaire — this time for a fundraising email he received from the Trump campaign.

"We are facing the FINAL end-of-quarter fundraising deadline before Election Day. I've set a $20 million goal that we must hit by the end-of-quarter FEC-mandated deadline," the email, a screenshot of which was posted by Fox on Twitter, said. "Now will you step up and help us meet our goal with a generous contribution?"

Over the course of his campaign, Trump has repeatedly said he would build a wall between the U.S. and Mexico, and "make Mexico pay" for it — comments that many Twitter users were quick to highlight.

"What a stupid advertising add (sic)," Fox said in his response to the email. "Shows extreme desperation and acceptance of defeat. Now you will pay for all your offenses and lies."

If the email was in fact sent by the Trump campaign, which is currently trailing Clinton's by roughly $50 million, it would be a clear violation of U.S. federal election laws that prohibit candidates from soliciting funds from foreign individuals, corporations and governments.

And this wouldn't be the first time the campaign's solicitation emails were sent to foreign email accounts. In June, government watchdog groups in Washington filed complaints with the Federal Election Commission against Trump's presidential campaign after reports emerged that lawmakers in the United Kingdom, Iceland and Australia had received fundraising emails. 

"Donald Trump should have known better," Paul S. Ryan, deputy executive director of Campaign Legal Center — one of the two organizations that filed the complaint — said in a statement in June. "It is a no-brainer that it violates the law to send fundraising emails to members of a foreign government on their official foreign government email accounts, and yet, that's exactly what Trump has done repeatedly. ... Trump's fundraising antics show that the FEC must also monitor candidates directly soliciting foreign money."