die Post
The totality of letters, parcels, and items being delivered.
Ich schaue mal in den Briefkasten, ob die Post schon da ist.
I'll check the mailbox to see whether the mail has arrived yet.

mail / post office
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The totality of letters, parcels, and items being delivered.
Ich schaue mal in den Briefkasten, ob die Post schon da ist.
I'll check the mailbox to see whether the mail has arrived yet.

'Die Post' in German primarily means 'mail' or 'post office'. The English word 'post' can also mean a pole, a job position, or a social media post. The German word does not have these meanings.
Ich warte auf wichtige Post von der Universität.
I'm waiting for important mail from the university.
When talking about the items being sent themselves, 'Post' is often used like an uncountable noun without an article. For example, you ask: 'Ist Post für mich da?' (Is there any mail for me?).
Ich habe heute noch keine Post bekommen.
I haven't received any mail today.
When referring to the service provider, typical phrases are 'etwas mit der Post schicken' (to send something by mail), 'ein Paket bei der Post aufgeben' (to send/drop off a package at the post office), or 'von der Post geliefert werden' (to be delivered by the postal service).
Ich schicke dir die Dokumente morgen mit der Post.
I'll send you the documents by mail tomorrow.
To describe the place, the prepositions are crucial. You go 'zur Post' (to the post office; movement, accusative), and you are 'auf der Post' or 'in der Post' (at/in the post office; location, dative).
Kannst du auf dem Weg nach Hause kurz zur Post gehen?
Can you quickly go to the post office on your way home?
The idiom 'die Post geht ab' is colloquial and means that things are very lively, loud, and exciting. It is often used for parties, concerts, or festivals.
Auf der Party gestern Abend ging echt die Post ab!
At the party last night, things were really happening!
program / shipment
'die Post' and 'die Sendung' can both have to do with letters and packages. 'die Post' often means everything that is delivered, or also the company that brings letters and packages. 'die Sendung' more specifically means a particular package, a letter, or several things that are sent to someone. Learners may confuse the words because both can be sent or received. A practical rule is: if you are talking about the system or about letters and packages in general, 'Post' fits. If you mean a specific item that has been sent, 'Sendung' fits better.

Heute kommt die Post wegen des schlechten Wetters etwas später.
Today the mail is coming a little later because of the bad weather.
Die Sendung für den Kurs liegt schon im Paketshop.
The shipment for the course is already at the parcel shop.