rentrez provides functions to search for, discover and download data from the NCBI's databases using their EUtils function.
Details
Users are expected to know a little bit about the EUtils API, which is well documented: https://www.ncbi.nlm.nih.gov/books/NBK25500/
The NCBI will ban IPs that don't use EUtils within their user guidelines. In particular
/enumerated
/item Don't send more than three request per second (rentrez enforces this limit)
/item If you plan on sending a sequence of more than ~100 requests, do so outside of peak times for the US
/item For large requests use the web history method (see examples for entrez_search or use entrez_post to upload IDs)