When the Justice Department wrote to Jackson, Miss., officials last March, reminding them of a 1976 Voting Rights Act decision disapproving the city's plan to annex some white suburbs, Rep. Trent Lott (R-Miss.) quickly sought out Deputy Attorney General Edward C. Schmults.

By July, Justice had dropped the objection.

In April, after a federal judge ordered Mississippi to improve conditions at its state prison, the Justice Department asked the judge to allow federal officials to inspect local jails as well. Lott asked Schmults for a three-week delay, and the state and the Justice Department worked out a compromise.

Then last month, after a mighty lobbying effort by Lott, the Reagan administration announced a policy shift to grant tax-exempt status to private schools that discriminate on the basis of race. This time, Lott's efforts partially backfired. The public outcry was so great that President Reagan introduced legislation under which future exemptions would be denied by Congress.

Lott has hounded the Justice Department so many times and on such a wide variety of subjects that some employes there now call him "our pen pal."

One source at Justice estimated that the Mississippi conservative, now the House Republican whip, sends them a letter every other day. "He runs the risk of overloading the circuits. When you send a letter every two days, people begin to discount them," he said.

Lott has also been very eager to share his views on legislative veto, which the Justice Department does not share. He believes Congress should have the authority to overturn federal rules and regulations to keep overzealous regulators under control.

"He has deluged us with missives on legislative veto," said a Justice Department source. "We must have had 10 different letters telling us why we were wrong, and we've written back 10 times telling him to stuff it."

Sources at the Justice Department say it is not unusual for congressmen to write or request meetings, but the frequency and ferocity of Lott's contacts have made him stand out, especially since he is not a member of the Judiciary Committee.

It is not clear how much influence Lott really has with the department. One department source says he believes Lott has worn out his welcome. "From the attorney general's standpoint, he kind of shrugs because Lott is everywhere . . . . He's written six-page memos on his views on the law. He's treated with a degree of amusement here."

Lott refused to be interviewed about his relationship with the Justice Department. But an aide says that Lott, whose position as whip makes him the second-ranking House Republican, has merely been trying to perform services for his constituents.

"Over his career, Mr. Lott has been willing to do just about whatever people ask him to do if it is in his power," the aide said, adding that Lott has just been more successful since the Republican administration took over. "There has been a noticeable shift since the Republican administration took over in their receptiveness to inquiries from members of Congress."

In an interview last summer with the Jackson, Miss., Clarion-Ledger, Lott said, "I haven't done anything other members don't do. I'm thankful I'm in a position where I can effect some change."

But, asked about his intervention in the pending prison case, despite a presumption of a separation of powers, Lott said, "Yes. I know, I know. But I just feel so strongly about that--that it is a policy decision that the Justice Department had to make whether to send agents into county jails. It's not an individual criminal matter. It's a policy decision. It's a fundamental federalist question. I acknowledge that it is unusual, probably."

Sources at the Justice Department say that Lott has enjoyed more access to the Justice Department than many congressmen because of one of his assistants, Michael B. Wallace, a 30-year-old Biloxi, Miss., lawyer who spent two years as a clerk to Supreme Court Justice William H. Rehnquist.

They say Wallace has a special pipeline to the department through lawyers there who also were Supreme Court clerks. For example, Charles Cooper, a special assistant to the head of the Civil Rights Division, and John Roberts, a special assistant to the attorney general, are both former Rehnquist clerks.

Wallace did not return phone calls from The Washington Post.

Wallace was hired early last year when Lott became minority whip. "Because of his experience with the clerkship, Mr. Lott felt he'd know how to go about accomplishing some of these things related to the Justice Department," said Lott's aide.

A staunch conservative, Wallace is said to be the author of some of the most fiery prose that reaches the department over Lott's signature. One department source added, "We've had a Wallace watch on since early on."

For example, department officials received a letter from Lott last Oct. 28, berating them for their handling of the Mississippi prison case. The letter, which is written on Lott's House stationery, has Lott's name typed at the bottom, but no handwritten signature.

Referring to discussions with Schmults last April, Lott said, "I was given to understand that it will not be the policy of Ronald Reagan's administration to force state penal authorities to conform to ideal standards. Rather, the department will seek adherence to only the minimum standards compelled by the Constitution.

"I have now been informed that the new lawyer in charge of the case, Mary McClymont, is insisting that the standards of the American Correctional Association concerning classification and medical care be applied to each of the county jails. I believe an explanation is in order," he wrote.

"I wish to know whether or not Mary McClymont is acting according to your wishes . . . . If she is, I want to know why administration policy has changed. If she is not, I want to know why she is being permitted to pursue her own policies at taxpayer expense. These are not rhetorical inquiries. I want to know, with reference to chapter and verse of the civil service statutes, why she has not been fired," he said.

"There are too many lawyers ready and eager to carry out Ronald Reagan's policies to permit those policies to be subverted by mere civil services," he concluded.

Sources familiar with the letter say that an attorney in the Office of Legislative Affairs intercepted it and warned Lott that it was too extreme. The letter was returned to him, but not before it had been copied and circulated.

McClymont has not been fired.

In addition to his efforts at the Justice Department, Lott has also urged the State Department to get rid of holdovers from the Carter administration. "Termites," he called them in an interview with the Clarion-Ledger, and said he had warned Secretary of State Alexander M. Haig Jr. that he should remove them or risk losing Republican support on foreign aid bills.

Lately, Lott has also taken an interest in the affairs of the Senate.

A Senate Judiciary subcommittee has held a number of hearings this month on extending the Voting Rights Act. And at almost every hearing, Lott's aide Wallace has been on the podium advising Sen. Orrin G. Hatch (R-Utah), the subcommittee chairman. A Democratic Senate source says that Wallace is serving as the "chief strategic adviser" to Hatch.

Since Lott is not a member of the House Judiciary Committee, he was not involved in putting together the House bill for extension, which passed last October by an overwhelming 389-to-24 vote. Lott voted against it.

Finally, the Mississippi congressman has stepped on some Senate toes since last summer when he began to lobby at the White House for an appointment to the 5th U.S. Circuit Court of Appeals. Judicial recommendations traditionally have been a Senate prerogative.

Sen. Thad Cochran (R-Miss.) recommended E. Grady Jolly, a Jackson lawyer, for the job, and Lott recommended Harry Allen, a former law partner from Gulfport, Miss.

Bill Minor, a veteran Mississippi journalist, says the competition over the judgeship is indicative of a "bitter rivalry" that has grown up between Lott, who ran Reagan's Mississippi campaign, and Cochran. Cochran is backed by party moderates while Lott heads the conservative wing of the party. The two men, the only two Republicans in the state delegation, came to the House together in 1973, and Cochran was elected to the Senate in 1978.

The judgeship situation has not been resolved, but Senate leaders have made it clear that they will not allow a House member to dictate a choice. Sen. Strom Thurmond (R-S.C.), chairman of the Senate Judiciary Committee, has passed the word that unless Cochran approves the choice, the judge will not make it through the committee.