When attaching single or multiple files, separating filenames and recipient addresses with "--" is mandatory, e.g:

$ mutt -a image.jpg  --  addr1  or mutt -a img.jpg *.png -- addr1 addr2.

The -a option must be placed at the end of command line options.

So one has to place a double dash between attachment and recipient. Took me a bit to figure that out, maybe it helps someone else here

mailx -s "Subject Here" -a backup.zip user@example.com

• -a is for headers
• -A is for files

Could you guide me on what really I need to do?

following are the commands and error message:-

~$ sudo apt-get install mailutils
sudo: error while loading shared libraries: libsudo_util.so.0: cannot open shared object file: No such file or directory
sudo: error while loading shared libraries: /usr/lib/sudo/libsudo_util.so.0: invalid ELF header