A career-change resignation letter explains that you are moving into a different field rather than to a competitor. That framing reassures your employer, protects the reference, and often opens the door to freelance or advisory work with them later.
Whatever your employment contract specifies — commonly two weeks to three months. If the contract is silent, statutory minimums in your country apply. Check the contract before you name a last working day in the letter.
You are not required to. 'Personal reasons' or 'a new opportunity' is enough. Detailed complaints belong in an exit interview, not in a permanent file document.
Usually yes, and it creates a timestamp, but attach the signed letter as a PDF. Many HR systems require the formal letter for the personnel file regardless of how it was sent.
No. Resignation is your decision. An employer can hold you to your contractual notice period, but they cannot refuse to accept that you are leaving.