“Come here immediately!”
When you hear the above command, will you come here
(a) at once?
(b) after 5 seconds?
(c) after 5 minutes?
(d) after 5 hours?
Of course, you will come here at once because you understand that “immediately” means without delay.
Similarly, if you read the following,
“My email requires your immediate action.”
You will surely take action immediately to see to the matter mentioned in the email.
Nonetheless, do not overuse it. Once, somebody received a sms from his sister requiring an immediate action to read an email. He was out and the phone was low battery. He did not know what the matter was but he was very anxious to know, maybe it was a life-or-death matter? He almost got a heart attack, I believe. But when he opened his email, despite the low-batt noise, his sister only wanted him to print a document.
If it is not as urgent as a life-or-death matter, the sister may want to rephrase the sms to “Need your help, please check email when convenient.”
On the other hand, you may want to put a due date, “Please settle the matter by tomorrow.”
Be careful of your choice of words, somebody may get heart attack because of you.