If Sue Flay's Cake Shop mails its acceptance to the Stan Still Stationery Store on June 5, and it arrives on June 7:
The acceptance could be nullified by a counter-offer which is delivered first to the Stan Still Stationery Store
The acceptance can never be effective on June 5
 The acceptance may be effective only upon receipt if the offer's terms required actual receipt
Under the mailbox rule, the acceptance is generally effective on June 5
If the acceptance was improperly dispatched, the acceptance would not be effective until June 7