If one receives mail on which the stamps were not postmarked in any way is one allowed to reuse those stamps to mail letters of one’s own? Is this considered stealing?
The stamps should not be reused.
Sources: Pe’as Sadecha (vol. 2, no. 159) presents this as an interesting question: Is a stamp that has inadvertently not been postmarked considered an aveidah, a matter of “lost property,” whereby it would be permitted if the postal service is of non-Jewish ownership, or is it considered gezel, theft that is prohibited even with respect to non-Jews. He asked this question to the Chazon Ish, who replied that he does not know for certain, yet stated that his leaning was that the stamp had become void, and its reuse would be gezel. Pe’as Sadecha concludes that the reasoning behind this would seemingly be on account of dina demalchusa, and questions what the halachah would be where dina demalchusa does not apply.