due to some security issues (Wannacry) Windows administrators are going to disable the unsecure smb1 protocol.
mount.cifs is using smb1 by default. So when you tried to connect to a Windows machine where smb1 is disabled you get an error: mount error(112): Host is down.
with virtualization it simply possible to change systems within a few mouse clicks. For example adding new disks or increasing the disk size of a hard disk without a reboot.
sometimes it occours that a systemd based Linux cannot rebooted.
the SMTP protocol is ASCII based. In the past, the SMTP protocol goes unencrypted over the wire means you can simply send email by connecting to the SMTP port 25 and enter some SMTP commands via telnet:
michael@debdev ~ # telnet smtp.yourdomain.org 25
This is a test message. End with <ENTER><ENTER> and point