Improve my if-statement to prevent double checks
Your code can be shortened to:

if(!array1.isEmpty() || !array2.isEmpty()) {
    mail.sendMail("Array1: " + getContent(array1) + ", Array2: " + getContent(array2));
}


When you have:

String getContent(ArrayList&lt;String&gt; list) {
    if(!list.isEmpty()) {
        StringBuilder sb = new StringBuilder();
        for(String string : list) {
            sb.append(string);
        }
        return sb.toString();
    }
    return "-";
}