Alert based in mail queue size for postfix and plesk forked from [agarzon's](https://gist.github.com/heartshare/7bdeebfab74ed436e72c163aadbb3004) and slightly tuned.

queueCounter.sh

#!/bin/bash -e

# Binary locations
readonly MAIL_BIN=$(command -v mail)
readonly QUEUE_SIZE=$(/usr/sbin/postqueue -p | tail -n1 | awk '{print $5}')
readonly QUEUE_SUMMARY=$(/usr/sbin/qshape -s deferred | head)
readonly CONTROL_FILE="/tmp/queueCounter"

# Comma-separated list of email addreses that should be notified when the
# queue grows above the defined threshold
MAILTO="mail@example.com,mail2@example.com"

# Queue size from which the script should start to send alerts
LIMIT=100

# Subject for the alert email
EMAIL_SUBJECT="WARNING: mail queue critical on $HOSTNAME"

# How many seconds the queue shoud be over LIMIT in order to send an alert
WARNING_DELAY=300

# Sends an alert email to every address specified on $MAILTO
send_notification_mail()
{
    IFS=','; for MAIL in $MAILTO; do
        echo "$QUEUE_SUMMARY" | $MAIL_BIN -s "$EMAIL_SUBJECT" "$MAIL"
    done
}

# Determines if a alert notification should be sent.
#
# Returns:
# - 1 no notification should be sent
# - 0 notification should be sent
should_send_notification()
{
    if [ ! -f $CONTROL_FILE ]; then
        return 1
    fi

    let next_warning_date=$(cat $CONTROL_FILE)+WARNING_DELAY
    let now=$(date +%s)

    if [ $now -lt $next_warning_date ]; then
        return 1
    else
        return 0
    fi
}

# Tries to delete the control file, if it exists
delete_control_file()
{
    if [ -f $CONTROL_FILE ]; then
        rm $CONTROL_FILE
    fi
}

# Creates a control fie, if it doesn't exist
update_control_file()
{
    if [ ! -f $CONTROL_FILE ]; then
        date +%s > $CONTROL_FILE
    fi
}

# Determines if the queue is above the defined alert threshold

# Returns:
# - 0 The queue is over the threshold
# - 1 The queue is not over the threshold
is_over_limit()
{
    if [ "$QUEUE_SIZE" -gt "$LIMIT" ]; then
        return 0
    else
        return 1
    fi
}

main()
{
    if is_over_limit; then
        if should_send_notification; then
          send_notification_mail
          delete_control_file
        fi

        update_control_file
    else
        delete_control_file
    fi

    exit 0
}

main "$@"