root@server:~# mailq

-Queue ID- --Size-- ----Arrival Time---- -Sender/Recipient-------
0528561D88      878 Wed Feb  1 21:46:12  root@example.com
                                         root@example2.com

0D14161E2B      657 Wed Feb  x xx:47:01  root@example.com
                                         root@example2.com

0798C61E0F      657 Wed Feb  x xx:45:02  root@example.com
                                         root@example2.com

14AF361E2F!     657 Wed Feb  x xx:48:01  root@example.com
                                         root@example2.com

root@server:~# mailq |grep "[[:alnum:]]!"
3658861E66!     657 Wed Feb  x xx:48:01  root@example.com
root@server:~# mailq |grep "^[[:alnum:]]!"
root@server:~#

3!
A!
c!

14AF361E2F!

$ mailq | grep -E '^[[:alnum:]]{10}!'