Is it like in the case of he/she was meant to write a recommendation letter for you but has forgotten? In which case I would just do something like this: Dear Sir/Madam, I hope I am not disturbing you with this email. The deadline for the application is approaching, and so I would greatly appreciate it if I could obtain the recommendation letter that you have agree to write for me from you. Kind Regards, NAME Hope this helps!

Like this: Please recall the recommendation letter we discussed the first week of May. ---Sincerely, Ju123