要するに
UNIXのメール送信機能
です。
試しにライブラリをのぞいてみましたが
# apt-cache show sendmail Package: sendmail Version: 8.15.2-8 ... Depends: sendmail-base, sendmail-bin, sendmail-cf, sensible-mda Suggests: sendmail-doc, rmail Description-en: powerful, efficient, and scalable Mail Transport Agent (metapackage) ... Sendmail includes *no* Mail User Agents (MUA), you'll have to pick from the plethora of available MUAs (mutt, vm, etc.) ||<
