Why the last few addresses can take longer

A list processes steadily, then slows near the end. The remaining rows look ordinary. Why are they taking so long?

An email check can depend on another server answering. Some checks finish quickly; others meet a delay, a temporary refusal or a timeout. If the service retries those rows, they can become the last unfinished part of the job.

This makes a completion estimate difficult. Half the rows finished doesn't necessarily mean half the time has passed. The remaining rows may be the slow ones.

More parallel requests don't automatically solve it. A receiving server can limit connections or refuse further checks. A system that keeps pushing can get less useful information instead of finishing sooner.

From the user's side, the useful distinction is whether work is continuing or has stopped. Look for a changing processed count, a retry state or a reported error. A progress bar alone may not explain the wait.

Avoid starting another copy of the same upload while the first is still active. Check its state first. If it appears stuck, contact support with the job identifier and when you submitted it; don't send the whole contact file unless it's needed.

Some checks will still end as unknown. Waiting longer can help with a temporary problem, but no amount of waiting guarantees a server will disclose whether a mailbox exists.