/home/<user>/.muttrc:
--- snipp ---
# Default sendmail call
set sendmail="/usr/bin/esmtp"
set envelope_from=yes
--- snapp ---


Give .esmtprc only the permissions 600!
/home/<user>/.esmtprc:
--- snipp ---
# Configuration file for ESMTP.

# Set SMTP host and service (port)
hostname = SMTPSERVER:25

# Set the user name
username = "USERNAME"

# Set the password
password = "PASSWORD"

# Use the Starttls
starttls = disabled
--- snapp ---