Guaranteeing the kashrut of food entrusted (מפקיד) or sent (שולח) via a non-Jew: seals (חבי״ת / חמפ״ג), what counts as a חותם, the siman of the ניקור, the trust (נאמנות) of the carrier, מירתת / יוצא ונכנס, two pots and the forgotten mold (Shulchan Aruch, Yoreh De'ah 118 — 13 se'ifim)
הַשּׁוֹלֵחַ יַיִן בְּיַד עוֹבֵד כּוֹכָבִים, אוֹ בָּשָׂר אוֹ חֲתִיכַת דָּג שֶׁאֵין בּוֹ סִימָן — צָרִיךְ שְׁנֵי חוֹתָמוֹת; אֲבָל יַיִן מְבֻשָּׁל וְשֵׁכָר וְחֹמֶץ וּמוּרְיָיס וּפַת וּגְבִינָה וְכַיּוֹצֵא בָּהֶן שֶׁאֵינָם אֲסוּרִים אֶלָּא מִדִּבְרֵי סוֹפְרִים — דַּי לָהֶם בְּחוֹתָם אֶחָד.
One who sends wine by the hand of a non-Jew, or meat, or a piece of fish without a siman, needs two seals; but cooked wine, beer, vinegar, מורייס, bread, cheese and the like, which are forbidden only מדברי סופרים, suffice with one seal.
Shulchan Aruch, Yoreh De'ah 118:1LEVEL 01
Hebrew text of the 13 se'ifim with a fluent English translation. One or two seals (חבי״ת / חמפ״ג), what counts as a seal, the siman of the ניקור, the trust of the carrier and מירתת explained with modern practical cases (tamper-evident seals, delivery, hekhsher, kitchen staff).
📖 StudyLEVEL 02
In-depth pilpul: the חבי״ת / חמפ״ג of the Taz sk1 (Rashi's דמיו יקרים / חיבת נסך vs the Rashba's דאורייתא / דרבנן), the Tosafot question of מפתח וחותם and ר״ת's answer, why ישראל חשוד is worse than the עכו״ם (Taz sk2), the נאמנות and the siman of the ניקור, חקירות and נפקא מינות.
📖 StudyLEVEL 03
Comparative tables (חבי״ת / חמפ״ג, שולח / מפקיד, one or two seals per product), golden rules, classic pitfalls (seal vs siman, ניקור, מירתת, two pots) and memorization of the 13 se'ifim.
📖 StudyLEVEL 04
The practical halacha according to the Shach, Taz, Pri Chadash and Pitchei Teshuva, then the Sephardic poskim (Yabia Omer, Yalkut Yosef) and Ashkenazi poskim (Aruch HaShulchan), applied to kosher seals, delivery and kitchen staff. Note: the Shulchan Aruch HaRav does not deal with this siman — this is a level of psak, not "Daat HaRav".
📖 StudyAccording to the Shulchan Aruch (YD 118:1), foods whose exchange would be a grave prohibition — wine, meat, a piece of fish without a siman — require two seals (שני חותמות); but what is forbidden only מדברי סופרים — cooked wine, beer, vinegar, milk, מורייס, bread, cheese — suffices with one seal. The Taz (sk1) sums it up with Rashi's mnemonic חבי״ת two seals, חמפ״ג one; the Rashba gives the reason: חבי״ת = a דאורייתא concern, חמפ״ג = דרבנן. The Rama adds that when sent by a non-Jew one seal suffices for everything (ר״ת), and bediavad we rely on it. For practical application to your situation, consult your Rav.
A thigh (ירך) sent without a seal is permitted if it is cut the way an Israelite cuts it after נטילת הגיד, or for any piece in which the ניקור of an Israelite is recognizable — the חוט דידא, the חוט שאצל החזה (se'if 5, cf. siman 63). By contrast a slaughtered animal or bird sent without a seal remains forbidden, since the mark of the שחיטה is not a reliable siman (se'if 6). The Taz (sk8) notes that the siman of the ניקור only helps בדיעבד, not לכתחילה. For practical application to your situation, consult your Rav.
According to the Shulchan Aruch (se'if 8), one who buys kosher meat and sends it via an עם הארץ — that carrier is נאמן, even if not מוחזק in kashrut: we do not suspect an exchange, and likewise the servants of an Israelite. A Yesh mi she-omer qualifies: one suspected of eating improperly permitted things is also suspected of exchanging. In practice seal, siman and נאמנות combine; the מפקיד who will see his own seal again fears an exchange more — hence traceability and responsibility. For practical application to your situation, consult your Rav.
According to se'if 10, a non-Jew left alone with kosher foods, even where an exchange would be a דאורייתא prohibition: if he comes and goes (יוצא ונכנס), or has stayed a long while without announcing that he would remain and without locking up — it is permitted, because he fears (מירתת) that the owner will come back and see him, even if the exchange benefits him. But if he announced that he would remain → forbidden. If he gains no benefit from the exchange → permitted in all cases (cf. ר״ס קכ״ט). For practical application to your situation, consult your Rav.
According to se'if 9, meat or cheese found at a non-Jew with a seal or with the word כשר written — even without knowing who wrote it — are kosher (we know they came from an Israelite), provided there are no non-Jews there who know how to write. The Rama permits buying cheeses sealed in the Israelite manner, only where there is no concern that the דפוסים (molds) remained accessible to non-Jews (יש אוסרים; bediavad one should not be stringent). This is the basis of the modern hekhsher as a חותם כשר: hence the certifying agency's control over the stamps, against forged labels. For practical application to your situation, consult your Rav.