Can u reverse it? Just create dictionary with keys (dates as Strings) and values as lists of Strings(emails)?
If yes u could build dictionary of lists of strings (call it dict):
Then for each email u are working with u can get date and put it to dictionary. Of course you should check if that date already exist in dictionary as a key. if not, we need to create empty list of string and then add email to it.
So it look like:
In above example in invoke method there is only one argument in (string) so that would be email.
After all if you would like to check if there is any emails in that dict with current date it will only need to check if dict.keys.contains(today.toString(“format of date”)) and if so, to get that list of strings (emails) from that key 