Is "keine E-Mails" plural?
Yes, it is.
Singular is for exactly one thing. For bigger numbers, use the plural form.
If the number is zero, it can get complicated. But if you indicate the number 0 with a form of the indefinite pronoun "kein" than you can choose freely between singular and plural. Both forms are correct:
Es gab kein E-Mail.
Es gab keine E-Mails.
Why "gab" instead of "gaben"?
Short answer:
Because "keine E-Mails" is not the subject. The subject is the word "es" which is in singular.
Here is a more detailed answer:
Lets turn the subordinate clause into a main clause and lets omit the adverb "früher" because is has no influence to our problem:
Es gab keine E-Mails.
There is the expletive "there" in English sentences, that is used to describe the existence of something:
There are mice in the basement.
There is a king in England.
There is the expletive "there" in English sentences.
There are no emails.
And in English you have the expletive "it" that is used to describe the weather:
It is raining.
It is snowing.
This expletive "it" is the subject of these sentences. You do exactly the same in German:
Es regnet.
Es schneit.
This expletive "es" or "it" is the subject, but it is not the thing that performs the action like subjects normally do and it does not refer to anything like personal pronouns do. So, these words are not pronouns in such constructions but just particles.
But German does not have a words that can be used the same way as the English expletive "there". For constructions, that are used to describe the existence of something, we also use the expletive "es":
Es gibt Mäuse im Keller.
Es gibt einen König in England.
Es gibt das expletive "there" in englischen Sätzen.
Es gibt keine E-Mails.
This German expletive existence-"es" always comes with a form of the verb "geben". (The English expletive "there" always needs a form of "to be" instead.)
But the main difference is, that the German expletive existence-"es" is alway the grammatical subject of the sentence. This "es" is always in singular (and it is always in nominative case because it is the subject). This means, that the verb in the sentence (which is always a form of "geben") also always must be in its singular form because it has to match with the subject in the grammatical number (singular/plural).
The verb geben needs not only a subject like all verbs, but also an accusative object. And this accusative object carries the information what exists. If only one thing exist, then this accusative object is in singular. If two or more things exist, it must be plural.
Note that in the English construction with the expletive "there" the situation is different. The word "there" is not the subject, but the thing that exists is the subject. So in the English version, the verb has to adjust its grammatical number to the number of the subject. But in German the verb is always in singular:
- English:
There is one mouse in the basement.
There are many mice in the basement.
There is no email.
There are no emails.
- German:
Es gibt eine Maus im Keller.
Es gibt viele Mäuse im Keller.
Es gibt kein E-Mail.
Es gibt keine E-Mails.