From the Web : It will take some time for the changes to take into effect.

[+Warwagon MVC]

The one thing I have never understood is why a company says things like "allows 24 hours for this to take effect" when unsubscribing from an email. I mean just remove my email address from your database or flip an attribute which says don't send this person an email, DONE! 

 

Other times I can sort of understand while it takes a while, like when switching Windows insider from the release ring to the fast ring, probably due to bandwidth/congestion or something. I otherwise it should pretty instant.

[Skiver Veteran]

It's Simple, replication of data is not instant.

 

Chances are it's not going to take 24 hours, it's probably going to take 5 minutes but this way they cover their ass if replication hasn't worked that change around its systems before whatever scheduled event kicks off the email blast etc.

 

Similar concept with an Active Directory, for example, I always tell users if I make a change that they need to allow 20-30 minutes for the change to take effect. In actual fact, I think the actual domain replication time is about 15 minutes for all DC's. This way I cover my ass so they aren't coming back to me because of their impatience and think it's not been changed right. Additionally, when it happens quicker it can give the view of efficiency because you exceeded their expectation in when said change would take effect. 

[+Nik Louch Subscriber²]

The server that processes the mailing action may be looking at a data warehouse, rather than the live subscription information.

 

But also, when a mass mail is sent - they are not all sent in one go.  They are staggered throughout the day to alleviate server load.  You may have mails in the queue so this covers that.