personalize mutt's subject with variable

personalize mutt's subject with variable

I've a script that send an email to user. I need to add a variable (name
of new user) to subject. My example:
a="John";echo|mutt -s welcome$a -e 'my_hdr From:info@example.com
<info@example.com>' test@example.com
Obviously this runs but the subject is: welcomeJohn. I need a space
between welcome and John. A real space doesn't works, change from a="John"
to a=" John" doesn'run... Either programmatically add a space to variable
or put all subject to variable.