I ran into this issue after moving some database server to a isolated network.
When booting your Debian machine and it hangs at “Starting MTA…” for a long time, it could be because you had your server online at some point and is now booting up without internet connection. Or has been configured as an internet server while being offline. If that the case, it is easy to fix. If your server is online and should stay online, check the nameserver ip is valid in resolv.conf
First thing you should do, is reconfigure exim4 (which is your MTA).
Just select local server, and go for default settings. Be warned that this will take a long time as it need to stop and start MTA again.
Next is to change the /etc/resolv.conf
And change to the following:
Try restarting your computer. The boot time should be massively improved.