Not long afterward, the Sanders campaign responded with a statement proclaiming "we have no idea if we will raise more money this quarter than the Clinton campaign."
But the campaign's guess: "Probably not."
The jockeying comes as both camps try to use whatever tactics they can to prod supporters to give before the Dec. 31 deadline and pad their respective year-end totals.
Sanders shocked Clinton and much of the political establishment by coming as close as he did last quarter. Clinton reported raising $29.4 million for the three months ending Sept. 30, while Sanders took in $26.2 million.
While Clinton maintains a solid lead in national polling, Sanders is showing a lot of life in Iowa and New Hampshire -- leading in most recent polls from the second nominating state. Both candidates were in the first caucus state on Tuesday, with Clinton making the rounds in eastern Iowa and Sanders campaigning on the western side.
Mook's email came on the same day that a Politico story appeared bearing the headline: "Clinton donors say Sanders is raising more than the front-runner." It quoted several Clinton boosters wringing their hands about about the prospect Sanders would collect more money this quarter.
In his email, Mook noted that Sanders has claimed recently to have received more donations than any candidate has at this point in a primary contest.
"The fact is, many of our supporters don't think this campaign needs any help," Mook wrote. "If you're one of those people, I hate to say it, but you're wrong. We're completely reliant on you and your support."
In its rejoinder, the Sanders camp said it had received 2.3 million contributions to date, averaging less than $30.
"This is greater than any candidate in history at this point in a White House campaign," the campaign said. "We are very gratified by that outpouring of grassroots support from middle-class and working families in this country."
The Sanders campaign also used the occasion to take a jab at Clinton for having several super PACs working on her behalf. Sanders has said repeatedly that he doesn't want the help of any super PACS -- groups that can raise and spend unlimited money -- though that hasn't stopped a couple from mobilizing to help him in a relatively modest way to this point.
"While we may or may not raise more money than the Clinton campaign, what is certain is that they will have more total money because they have established super PACs which are raising money from millionaires and billionaires," said the Sanders campaign.