The mailman delivered a letter from a mustard company, which brought up a question: Why would the mustard moguls spend 13 cents to reach the sports department? Perhaps they had invented a way to eat a ball park hot dog without suffering mustardial runoff. Or, speaking of hot dogs, maybe they had signed Reggie Jackson for some television testimonials.

It was, thank goodness, better news than that. The mustard company, on behalf of a wine company, conducts a weekly poll of sportswriters to pick the best college basketball team in the East. So all Georgetown University fans ought to run out and buy some mustard to mix with their win, because in this week's poll the Hoyas are ranked second in the East, behind Syracuse and six notches ahead of St. John's, tonight's opponent at McDonough gymnasium.

"Georgetown, unranked last week, vaulted into the No. 2 spot by virtue of its upset victory in the Holiday Festival in New York's Madison Square Garden," the mustard letter said. These are good times for Georgetown, the mustard vote coming so soon after the Hoyas' appearance in the Establishment ratings - 15th in United Press International, 20th in Associated Press. But the coach, John Thompson, is claiming nothing yet, not even the mustard championship of the East.

"Like last year," he said gently. "When we played Alabama last year, we were ranked 17th and they were No. 4. We lost to Alabama by two points - and we dropped out of the polls. Now, you'd think if the 17th best team lost to the would move up. We dropped out of sight."

Thompson smiled. The polls, he said, are good for talking and bragging by students and alumni. They build interest and recognition. That's good. But on the basketball court, they don't count. "Teams rate themselves at the end of the year," he said, meaning there's a national tournament to settle all disputes. "Polls are just somebody's opinion."

And opinions come cheaper than mustard. Across America, for instance, opinion holds that Georgetown University basketball is a cut beneath the big boys. Never mind that Georgetown made it to the NCAA Tournament in 1975 and 1976, then was in the National Invitation Tournament last season. We're talking about image, sports fans, and if the University of Maryland gets the bigtime ink, that means nothing either, for there seems little doubt that Georgetown today is the best team in own.

Maryland beat Georgetown, 91-87, in the championship game of the Tip-off Tournament a month ago. But georgetown's Craig Shelton broke his hand in that one, an injury that caused him to miss the next five games. And now, at a time when Maryland is losing at home to an ACC team that had won only one conference road game in its last 31 tries, Georgetown may be ready to play its best basketball of all.

"We definitely can be better than we've been so far," said Thompson, whose team has a 9-2 record and is on an seven-game winning streak. "We've just had so many nagging injuries that the team hasn't been able to settle yet. At the Holiday Festival, for one in uniform. We've played some games with seven guys."

Victory in the Holiday Festival was uncontestable evidence that Georgetown is for real. First it beat Holy Cross, until then underfeated. For the championship, Georgetown, which plays in a 4,400-seat gymnasium with wooden bleachers, went against Alabama, whose home is a stately pleasure domes with 16,000 seats. Too much can be made of such a match, the "have-nots" against the "haves", but then too little can be made of it, too, for the fact is that a basketball team with a good coach and three or four exceptional players can beat any team. The only image that counts then is that in lights on the scoreboard: Georgetown 83, Alabama 73.

"It was good for the kid's self-confidence and belief in themselves." Thompson said. "The polls recognized them, too. To me, it is more enjoyment, though, to win a tournament, because that is an actual accomplishment, not an opinion."

Nor is it unfounded opinion that Georgetown's guards are playing superbly. Derrick Jackson, a senior, is scoring 19.9 points a game, and sophomore John Duren 16.3. In his last three games, including two in the Holiday Festival, Duren has made 26 of 36 shors. "He can wind up being one of the better players in the country," Thompson said. "And Derrick has held this program together the last four years."

The Georgetown starting forwards are Shelton, at 6-foot-7 an exceptional athlete, and Steve Martin, at 6-4 1/2 the team's leading rebounder and second to Duren in assists The starting center tonight will be Tom Seates, a 6-11 junior whose scowl will frighten away any enemy shot that he doesn't block first.

Two of last year's starters, A1 Dutch and Ed Hopkins, have done outstanding jobs off the bench, Thompson said, and with guard Mike Riley give the Hoyas essential depth. "It gives us a high level of competence," the coach said.

On last thing. At New York, a tournament official was delighted with the Holiday Festival teams. "He said all of the teams, except for Georgetown, were ranked in the Top 20," Thompson said. "He didn't mean anything by it, but that kind of thing gets the adrenalin to going. It makes you feel like a stepchild. Things like that made me want to play."

Pass the mustard.