Install packages.

apt install arpwatch msmtp

Configure arpwatch interface.

cat > /etc/arpwatch/eth0.iface <<'EOF'
IFACE_ARGS="-s /usr/bin/msmtp -m monitor@1k2.nl"
EOF

Configure msmtp (Fastmail example).

cat > /var/lib/arpwatch/.msmtprc <<'EOF'
defaults
auth           on
tls            on
tls_trust_file /etc/ssl/certs/ca-certificates.crt
logfile        ~/.msmtp.log

account         fastmail
host            smtp.fastmail.com
port            465
from            <FROM ADDRESS>
user            <YOUR USERNAME>
password        <YOUR APP PASSWORD>
tls_starttls    off
set_from_header on

account default : fastmail
EOF

Set owner and mode:

chown arpwatch. /var/lib/arpwatch/.msmtprc
chmod 600 /var/lib/arpwatch/.msmtprc

Start arpwatch at boot.

systemctl enable --now arpwatch@eth0