Iowa Code § 504.807: Resignation of directors.

1. A director of a corporation may resign at any time by delivering written notice to the board of directors, its presiding officer, or the president or secretary.

2. A resignation is effective when the notice is effective unless the notice specifies a later effective date. If a resignation is made effective at a later date, the board may fill the pending vacancy before the effective date if the board provides that the successor does not take office until the effective date.