A possible explanation for the tactic of sending so many emails is that the subject header of Donald Trump's emails contained the message he wished to transmit,

A possible explanation for the tactic of sending so many emails is that the subject header of Donald Trump's emails contained the message he wished to transmit, acting like an alternative news feed, similar to RSS or Twitter, bypassing the restrictions of the latter. In other words, recipients would not be required to open the email to get the message.