The Tehuantepec treaty with the United States has been rejected by the Mexican Congress. The details of this action, which can not fail to be considered as highly important to this country, have not reached us. ---- From the city of Mexico we have dates directly only to the 5th of March. The embassadors of Great Britain, France, Spain, and the United States have addressed a remonstrance to the Mexican government against the unfairness of the custom-house regulations in Mexico. The Mexican Secretary has replied, that the matter is before Congress, and that it does not call for any interference on the part of foreign ministers. Tuspan has been made a port of entry. ---- A contract has been entered into by the King of Belgium and the Mexican Government, for transporting 50,000 Belgians to the interior of Mexico, where they are to receive lands to settle on, or work for Mexican landholders, on certain stipulated conditions. ---- A bill has been introduced into Congress repealing the stringent laws concerning foreigners, and imposing the penalty of banishment on any foreigner who may be judicially convicted of taking part in any revolutionary government, of having abused the liberty of the press, or of smuggling. At present foreigners may be expelled simply on suspicion, and without any judicial inquiry whatever. ---- A letter from Louis Napoleon, announcing the change in the government of France, to his "great and good friend," the President of the Mexican Republic, is published in the Mexican papers. ---- Complaints are constantly made against the Mexican authorities at Acapulco, of maltreatment of Americans, and insults to the American flag. Great numbers of emigrants to California have been driven into Acapulco by wreck and other causes, and they very frequently come into conflict with the local officers. Two or three instances are mentioned in which Americans have been imprisoned on the most frivolous pretexts, and the remonstrances of the U. S. consul treated with contempt. GREAT BRITAIN.