1. Classification of the 4 different types of mail slot openings (exterior, interior, thru-wall (e.g. fence or wall), door sides or side walls).
2. Identification of 2 different mail slot sizes (325–400 mm for vertical insertion of test envelope and 230–280 mm for horizontal insertion, mail slot height 30–35 mm).
3. Resistance capacity against corrosion according to DIN EN 1670 (pertains principally to locks and other installed materials, to verify this a 240 hour salt spray test, among other things, is used) and against water penetration (to test this a special sprinkling system is used, maximum penetration allowed is 1 ccm).
4. Resistance to theft through use of stable materials and locks with 2 security levels.
5. Guarantee of the minimum size of a mailbox: A test envelop in format C4 must fit, without being folded or damaged. This way the proper delivery of normal parcels is guaranteed. The test envelope is ca 24 mm thick. The minimum volume is established as a pile height of at least 40 mm postal goods in format C4.
6. The midpoint of the mail slot should be between 700 mm and 1700 mm, in cases of exception the midpoint can be between 400 mm and 1800 mm. That is a regulation on the constructor in order to ease the work of the mailman.
7. To guarantee safety, the components of the mail slot may not have sharp corners. That minimizes the danger of injury to the mailman.
8. To guarantee privacy, the mailboxes must be equipped without view window.
9. Retrieval security against meddlers. For this, the mailboxes must be equipped with retrieval security, which, depending on the mailbox type, must have a minimum size and a fixed inclination in the insertion direction of the mail.

This norm is a recommendation, which however, through inclusion in tenders, becomes an obligation.
Moreover, attention should be paid to the fact that litigation is always decided upon the current technical status – that is, the corresponding norm (see examples from court decisions).

The advantages to purchasing a mailbox manufactured to the
DIN EN 13724 standards:

• no crinkled, folded or otherwise damaged letters and newspapers,
• no long way to the post offi ce to pick up big, undeliverable parcels, (The mailman is authorised to refuse to deliver to non- EN-compliant, in order not to damage the mail. In these cases he leaves a note in or on the mailbox.)
• no wet mail by appropriate delivery (complete insertion of letters, newspapers and advertisements)
• no justified rent reduction (see below)
• best quality against rust through fi xed corrosion-resistant materials
• optimum security against vandalism and theft of mail through fixed security standards

Verdicts:
Smaller Mail Slot, Lower Rent!
Cologne (AP): If the slot in the mailbox isn‘t big enough in order to receive DIN A4 format mail, the tenant may reduce the rent by a half procent. The Berlin Regional Court decided: „If problems with the delivery of magazines or DIN A4 envelopes occur as a result of the mailboxes installed by the landlord, a monthly reduction is appropriate.“ (File No. 29520/90)

Wet Mail, Lower Rent!
A mailbox belongs to the furnishing of a rented flat. If it is difficult to open and the mail becomes wet in the rain, then there is a deficiency which justifies a reduction of one percent of the rent. (Local Court of Mainz, 8 C 98/96)