Looking after a large list
A large list without maintenance turns into a list of dead addresses.
As the list grows, some address dies. Without a routine, you only find out when a job fails halfway.
The routine
Run the list through the testing tool once a day. Whatever does not open its port goes to replacement, which swaps one address at a time every 24 hours. Note what was swapped, because the new IP may be on one of your access lists.
On renewal, decide beforehand: keeping the same addresses preserves the list; not keeping them delivers a new list and resets the maintenance work.