From the Detroit Free Press:

Do you have a completed absentee ballot, but haven’t put it in the mail yet? You need to act immediately — and probably avoid the mailbox altogether — if you want to ensure your vote is counted, said Michigan Secretary of State Jocelyn Benson. 

Although the election is next Tuesday, Aug. 4, only those ballots received by the local clerk by the time polls close on Election Day will count. Problems or delays with the mail may prevent the clerk from receiving the ballot in time. 

“This close to the primary, Michiganders should return their absentee ballots to their clerk’s office directly, or submit them via their local ballot drop box, in order to ensure their vote is counted,” Benson said in a news release.