Short Answer A defect notification ("Mängelanzeige") is legally valid in any form — email included — because German law attaches your rights to the landlord's knowledge of the defect, not to paper. The trap is proof: a notification you cannot prove was received protects nothing, which is why the registered letter remains the weapon of choice for anything expensive. What Most Expats Don't Realize You sent three polite emails to your landlord over a two-month period requesting remediation for a major water leak forming behind your bathroom tiles. The landlord ignored them while the moisture spread into your living room walls — and when you finally escalated, he claimed the emails had never arrived and accused you of concealing the damage for months. Unable to prove when your notifications landed, you watched your retroactive rent reduction evaporate and faced a €900 repair charge for the alleged late report. What To Do * Send a formal defect notice ("Mängelanzeige") with a clear deadline via registered mail with return receipt the moment a serious defect appears — not because email is invalid, but because the postal receipt is evidence the landlord cannot deny. * State explicitly in the document that you are paying your future monthly rent strictly under reservation ("unter Vorbehalt") to keep every retroactive claim alive. * "Hiermit fordere ich Sie auf, den Mangel bis zum genannten Datum zu beheben." (I hereby demand that you remediate the defect by the specified date.) — use this exact phrase to set a legally binding two-week execution window. The Truth Germany's tenant-landlord law is less formalistic than its reputation — your rights arise the moment the landlord knows, however he came to know it. The system's real currency is provability: landlords ignore friendly channels precisely because an unprovable notification is legally indistinguishable from silence.