Question: How to trigger a email when value of custom field is updated?

Answer: Create a notification as shown in the screenshot below:


Add message template similar to the example below:

#foreach ($item in $issue.lastchange.items)
	#if ($item.fieldId == "CAB" and $item.fromString == "value before" and $item.toString == "value after")
		<pre>
		Hi there,

		User $issue.lastchange.author.displayName has made some changes to <a href="$context.baseurl/browse/$issue.key">$issue.key</a>:

		<table border="1">
	    <tr>
    	    <th><b>Field</b></th>
        	<th><b>Old value</b></th>
	        <th><b>New value</b></th>
    	</tr>
	    #foreach ($item in $issue.lastchange.items)
        <tr>
            <td>$!item.field</td>
            <td>$!item.fromString</td>
            <td>$!item.toString</td>
        </tr>
    	#end
		</table>

		Best,
		Raley
		</pre>
	#end
#end

Make a note of the code in line 2 : it shows how to check the customfield value against before and after.