On Sunday, SPIEGEL reported that America's National Security Agency (NSA) had accessed the email system of Mexico's "Presidencia" domain, believed to be used by members of former President Felipe Calderon's cabinet.
Mexican authorities responded quickly, saying the same day that they would be seeking answers from US officials "as soon as possible."
"This practice is unacceptable, illegitimate and contrary to Mexican law and international law," Mexico's Foreign Ministry said in a statement. "In a relationship of neighbors and partners, there is no room for the kind of activities that allegedly took place."
Last month, the Brazilian Globo TV network revealed that a document dated June 2012 indicated the NSA had read current Mexican President Enrique Pena Nieto's emails before he succeeded Calderon in December 2012.
At the G20 meeting in Russia last month, Obama promised Nieto to carry out an "exhaustive investigation" into who was responsible for the suspected espionage.
"Mexico will re-emphasize the importance for our country of this investigation, which should be concluded as quickly as possible," the ministry said in its statement.
A Relatively Muted Response
Mexico is one of the United States' biggest trading partners and the latest claims could damage ties as the two sides seek to improve cooperation on issues like cross-border security, migration and fighting organized crime.
But as a country that sends nearly 80 percent of its exported goods to the US, Mexico's response to the spying allegations has so far been more muted than Brazil's.
After the Globo TV report alleged that the NSA had also snooped on her communications, Brazilian President Dilma Rousseff suspended plans for a state visit to Washington and later blasted the US over spying at the UN General Assembly.