Usually i get a certain percent of support tickets related to sending issues and first thing that i do is to look in Backend > Miscellaneous > Campaigns Delivery Logs.
This area contains a lot of information that tells you exactly how your sending works, if there are errors, and if there are, what type of errors.
Most of the time, you’ll see errors where the remote server doesn’t accept connections from your server anymore, or that i screwed things with an update and i broke some implementation for certain type of delivery server(it happens, we’re all humans), or for any other various reasons, regardless of which, they are all listed in the logs area and you should look at that area often enough to make sure your app works properly.
Same is true for the bounce logs area from Backend > Miscellaneous > Campaigns Bounce Logs.
Bottom line, when in trouble, read the logs, the solution of the problem might as well be listed there