cd /etc/postfix/maps
ln virtual_alias_maps.txt virtual_mailbox_maps.txt
postmap virtual_mailbox_maps.txt
postconf -e 'virtual_mailbox_maps = hash:/etc/postfix/maps/virtual_mailbox_maps.txt'
rm virtual_alias_maps.txt virtual_alias_maps.txt.db

¹⁾

cp -a /usr/share/doc sonntag

cp -al sonntag montag
rsync -a --del /usr/share/doc/ montag

cp -a montag dienstag
rsync -a --del /usr/share/doc/ dienstag

…

 |-- dir1
 |   |-- a
 |   |   |-- file1.txt
 |   |   `-- file2.txt
 |   |-- b
 |   |   `-- file3.txt
 |   `-- c
 |       `-- file4.txt
 `-- dir2
  |-- a
  |   |-- file5.txt
  |   `-- file6.txt
  |-- b
  |   |-- file7.txt
  |   `-- file8.txt
  `-- c
   |-- file10.txt
   `-- file9.txt

mkdir merge
cp -rl dir1/. dir2/. merge
rm -r dir*
tree merge 

merge
├── a
│   ├── file1.txt
│   ├── file2.txt
│   ├── file5.txt
│   └── file6.txt
├── b
│   ├── file3.txt
│   ├── file7.txt
│   └── file8.txt
└── c
    ├── file10.txt
    ├── file4.txt
    └── file9.txt

²⁾