Many client agreements will contain provisions that allow for (or require) one party to deliver a demand or notice to the other party. The exact date those demands/notices are delivered will usually start some kind of daily countdown.
For that reason, many contracts will include a notice provision that says
(a) where demands and notices must be sent; and
(b) when they will be deemed delivered.
The address (physical or email) provided for each party in the contract is the address where demands/notices must be sent. If a party changes their address, they can send a notice to the other party to update the address applicable for them for future notices.
There’s so much more to learn! Here are a few related guides you should read: