Use debian buster in Vagrantfile

Vagrantfile

@@ -2,7 +2,7 @@
 # vi: set ft=ruby :
 
 Vagrant.configure("2") do |config|
-  config.vm.box = "debian/stretch64"
+  config.vm.box = "debian/buster64"
 
   config.vm.synced_folder "./", "/vagrant", type: "rsync"
 

vagrant-util/send-test-mails.sh

@@ -16,14 +16,14 @@ if [[ "$1" != "--LETS-REALLY-DO-THIS-AND-SEND-MAIL" ]]; then
 fi
 
 # Simulate generic emails
-echo "(1) This is a very plain and simple email for testing purposes." | mail -s "First testmail" -r localsender1@example.com first-mail@example.com
-echo "(2) This is a very plain and simple email for testing purposes." | mail -s "Second testmail" -r localsender2@example.com second-mail@example.com
-echo "(3) This is a very plain and simple email for testing purposes." | mail -s "Third testmail" -r localsender3@example.com third-mail@example.com
-echo "(4) This is a very plain and simple email for testing purposes." | mail -s "Fourth testmail" -r localsender4@example.com fourth-mail@example.com
-echo "(5) This is a very plain and simple email for testing purposes." | mail -s "Fifth testmail" -r localsender5@example.com fifth-mail@example.com
+echo "(1) This is a very plain and simple email for testing purposes." | mail -s "First testmail" -r localsender1@example.local first-mail@example.local
+echo "(2) This is a very plain and simple email for testing purposes." | mail -s "Second testmail" -r localsender2@example.local second-mail@example.local
+echo "(3) This is a very plain and simple email for testing purposes." | mail -s "Third testmail" -r localsender3@example.local third-mail@example.local
+echo "(4) This is a very plain and simple email for testing purposes." | mail -s "Fourth testmail" -r localsender4@example.local fourth-mail@example.local
+echo "(5) This is a very plain and simple email for testing purposes." | mail -s "Fifth testmail" -r localsender5@example.local fifth-mail@example.local
 
 # Simulate emails with attachments
-echo "Let's try an email with an attachment" | mail -s "This email contains an attachment" -r attachments@example.com -A /vagrant/main.go files@example.com
+echo "Let's try an email with an attachment" | mail -s "This email contains an attachment" -r attachments@example.local -A /vagrant/main.go files@example.local
 
 # Simulate emails for multiple recipients
-echo "I send to multiple people at once" | mail -s 'Hey all!' -r 'multirecip@example.com' one@example.com two@example.com three@example.com four@example.com five@example.com
+echo "I send to multiple people at once" | mail -s 'Hey all!' -r 'multirecip@example.local' one@example.local two@example.local three@example.local four@example.local five@example.local