I want to use mutt & esmtp to send e-mail from my machine. However when I try to send an e-mail, I get this error message:

open: (null): No such file or directory
and
Output of the delivery process -- (all)
Error sending message, child exited 78 ().

my .esmtprc file looks like this:
hostname="mailserver:25"
username="myuser"
password="mypassword"

my .muttrc contains this line:
set sendmail="/usr/bin/esmtp -C ~/.esmtprc"

However even if the e-mail is not sent, it is copied to my INBOX.Sent IMAP directory correctly, so no problem with that.

Is it possible to get some kind of traces of what mutt or esmtp is trying to do? Or can you help me resolve this problem?
Thanks in advance.

I tried to append -X /tmp/esmtp.log to esmtp in .muttrc but the log file is empty even after mutt fails to send the e-mail, so I guess it does not even get to the point of sending e-mail.