you need to speak to whoever administers your email server
it is their responsibility to deliver emails in a timely manner.
this has nothing to do with wppizza or indeed anything the plugin (i.e: I) could do anything about

again, you need to speak to your mail administrator, the plugin has no influence over this .

it simply sends an email (programmatically) and the mail server has to deal with it.
at that point it is completely the mail servers responsibility to do whatever it is it needs to or wants to do .

there might be all sorts of things that will influence the delivery (mail headers, spam settings, server setup in general etc etc ) but all of that is completely out of the plugin’s hands