Sendmail is the traditional MTA (message transfer agent), i.e. the traditional server program to send, transfer and deliver email. If your question is about sending mail in general, see the tag /email.
Sendmail is the traditional unix message transfer agent (MTA). It listens on remote smtp connections for incoming emails on smtp connections, handles local delivery (possibly with the help of an MDA, and sends locally-originating email.
Use this tag if your question is about Sendmail. If your question is about another MTA, use the tag for that MTA instead. If your question is about sending mail in general, see the tag email.