Neither snow, nor rain, nor heat, nor gloom of night used to prevent the postal couriers from delivering the mail, and now the Postal Service says it will not be deterred on its appointed rounds by a nuclear war, either.

But a 300-page plan for continued mail deliveries after an atomic war or other national emergency was ridiculed today by members of the House Post Office and Civil Service Committee's Subcommittee on Postal Personnel and Modernization and by others who called the plans ''idiotic,'' ''deceitful'' and ''futile.''

Under skeptical questioning from committee members, Ralph H. Jusell, civil defense coordinator for the Postal Service, conceded that the plans would not work in the event of an ''all-out, preemptive'' nuclear exchange between the United States and the Soviet Union. But he said the plans would probably be effective in a limited nuclear conflict.

According to Mr. Jusell and James K. Jones, general manager of the Postal Service's Prevention and Planning Division, the goals of the contingency plans are to deliver mail in an emergency ''to the extent possible under the circumstances'' and to protect postal employees as much as possible. Headquarters Transfer Planned