There are data extensions. I am sending emails by selecting the whole list. But email is being sent to only half or less than half in the list. What could be possible reasons?
It is to little information and a description that is not precise. I encourage you to rephrase your question and give as much information as you can do without showing customer data. Especially the description with dataextensions and lists is quite bad formulated. Lists and dataextensions are two completely different things.
Contacts that have an unsubscribe status inside that list and you try to send with a commercial sender profile could be one.
Duplicate email addresses all over the place with the option "deduplicate subscribers" inside the normal send process (all duplicate email addresses get pushed out).
Contacts that are inside the suppression period / pending deletions.
Exclusions scripts that you might have set.
Throttling processes that have been missconfigured and a dataextension that gets overwritten meanwhile that process.
A different user that stops the sending process when its scheduled and not finished (still processing the sends).