You should learn more about the mail client you use instead of relying 
on how someone configured it for you. IMP allows configuring a 
different sent mail folder for each identity, additionally you can 
change it in the compose view. With newer versions you can even bind 
identities to email addresses other than the identity's From: address. 
This should give you enough flexibility.

Interestingly, I can't see that on my compose view (though am using it 
currently through cPanel, with no control over its configuration). I 
see only a checkbox for saving it in sent-mail.



Just as interestingly I can't find which precise version of IMP I'm 
using. It's not visible anywhere obvious.



All the same, it's dead easy to make such an option automatic, and the 
less selection of anything I have to do as an end user is attractive. 
A long time ago I had a mailer configured just to ave all outgoing 
emails and all incoming emails once read, by default in a folder the 
name of which was taken from the From header. The end result is an 
archive of emails to and from any one address in on folder per 
address. It seems elegant features like this aren't as common any more?

Isn't it enough that you can select the sent mail folder in the 
compose view if you configure IMP that way?

For the inbox it's fine to have sent mail stored in a global sent-mail 
folder or somewhere. When working is specially themed folders though I 
generally want for my outgoing mail to be stored in the same folder as 
the the mail I'm replying to. Outlook has such an option and it's very 
useful if this happens automatically.