Ideally, Check_MK uses /usr/sbin/sendmail to send notifications, everything else
depends on the local MTA.
However,there are two active checks configurable via WATO:
Here you can specify the SMTP server
Check Email Delivery
This active check sends out special E-Mails to a defined mail address using the SMTP protocol and then tries to receive these mails back by querying the inbox of a IMAP or POP3 mailbox. With this check you can verify that your whole mail delivery progress is working.
Check SMTP to access services
This check uses check_smtp from the standard Nagios plugins in order to try the response of an SMTP server.