While most stamps are now self-adhesive, most envelopes still require moistening the adhesive in order to seal it. Is it a kashrus problem to lick the envelope's adhesive?
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Per dinonline there is no Kashrus concern with licking an envelope.
Per Halacha Berura - Rabbi Belsky - Page 3 there is no concern during the year, however on Pesach one should be stringent since the paste may contain wheat starch.