You may find it helpful to remember that 'the' is definite - and for something that's definite you have one and only one option.
If 'the' does not represent 'one and only one' then it's use is probably optional. In your 2 proposed sentences the first one has one and only one recipient - we need 'the' in the sentence, it doesn't sound right without it.
In your second option there are multiple Account Managers and the the sentence sounds correct without 'the'. Using 'the' isn't incorrect here, it still sounds correct, but it also sounds correct without it.
Does that help you?
The definite and indefinite articles are a form of 'determiner' -- a word used to identify which particular thing or things you are referring to. In general if the noun is plural (so "Account Managers" in the second question) then the use of a determiner is optional. That doesn't mean that using it is wrong.
There's more information on determinars in this link.
In general when we communicate we should intend be to be clear in our meaning, and as determiners help us to be more specific usually using them is helpful.
To illustrate this, consider these variations of the question's second sentence with an introductory sentence.
Introductory text: Account Managers of under performing accounts neeed to be aware of the problem. We have decided that for low engagement subscriptions we will be ...
... putting a placeholder to send emails to the Account Managers.
... putting a placeholder to send emails to those Account Managers.
... putting a placeholder to send emails to Account Managers.
These all convey the same meaning, all are correct, however some will feel more natural than others to certain speakers.