The default size parameter (number of records actually returned) is 10 - https://www.elastic.co/guide/en/elasticsearch/reference/current/search-request-from-size.html
If you change your query to GET exchangemails/email/_search?_source=sender&size=50 then you'll get 50 records.
/exchangemails/email/_search?_source=sender&from=50&size=50 will then get the next "page" of results.
The max supported value for size is 10,000, and while this can be changed system wide, utilizing scroll (https://www.elastic.co/guide/en/elasticsearch/reference/current/search-request-scroll.html) or search after (https://www.elastic.co/guide/en/elasticsearch/reference/current/search-request-search-after.html) are generally accepted as better solutions