Rodando
aptitude search '~Pmail-transport-agent' | grep "^i"
retornará o seguinte se não houver um pacote instalado que forneça um agente de transporte de e-mail adequado
i lsb-invalid-mta - Linux Standard Base sendmail dummy
ou seja, se você executar
aptitude search '~Pmail-transport-agent'
você receberá todos os pacotes que eventualmente poderão enviar e-mails. No caso do Ubuntu 16.04 que é
p citadel-mta - complete and feature-rich groupware server (mail transport agent)
p citadel-mta:i386 - complete and feature-rich groupware server (mail transport agent)
p courier-mta - Courier mail server - ESMTP daemon
p courier-mta:i386 - Courier mail server - ESMTP daemon
p dma - lightweight mail transport agent
p dma:i386 - lightweight mail transport agent
p esmtp-run - user configurable relay-only MTA - the regular MTA
p exim4-daemon-heavy - Exim MTA (v4) daemon with extended features, including exiscan-acl
p exim4-daemon-heavy:i386 - Exim MTA (v4) daemon with extended features, including exiscan-acl
p exim4-daemon-light - lightweight Exim MTA (v4) daemon
p exim4-daemon-light:i386 - lightweight Exim MTA (v4) daemon
i lsb-invalid-mta - Linux Standard Base sendmail dummy
p masqmail - mail transport agent for intermittently connected hosts
p masqmail:i386 - mail transport agent for intermittently connected hosts
p msmtp-mta - light SMTP client with support for server profiles - the regular MTA
p nullmailer - simple relay-only mail transport agent
p nullmailer:i386 - simple relay-only mail transport agent
p opensmtpd - secure, reliable, lean, and easy-to configure SMTP server
p opensmtpd:i386 - secure, reliable, lean, and easy-to configure SMTP server
p postfix - High-performance mail transport agent
p postfix:i386 - High-performance mail transport agent
p qmail-run - sets up qmail as mail-transfer-agent
p sendmail-bin - powerful, efficient, and scalable Mail Transport Agent
p sendmail-bin:i386 - powerful, efficient, and scalable Mail Transport Agent
p ssmtp - extremely simple MTA to get mail off the system to a mail hub
p ssmtp:i386 - extremely simple MTA to get mail off the system to a mail hub