No, it’s just a service that’s running without me thinking about it.

My setup is:

• debian as system
• exim for smtp
• courier for pop3
• mariadb for accounts

But I’d like to make a point that’s not being made in any of the other comments. It does not require an SMTP server to send e-mail. All you have to do is lookup the MX DNS record of the domain, connect to that SMTP server and write a few commands fx.:

EHLO senderdomain.tld
MAIL FROM:<yourmail@yourdomain.tld> 
RCPT TO:<recipient@recipientdomain.tld>
DATA
Subject: Blabla

Bla bla
.