Quick answer:
Greylisting.
Long answer:
Read this:
or summarised:
Greylisting: Greylisting refers to a form of spam control for emails, in which the first email from unknown senders is initially rejected and only accepted after another delivery attempt.
Consequently, you can do little as the sender because the review takes place namely at the destination address (Google, Hotmail, etc.). And the duration of each mail delivery then depends on how quickly the second check of the mail server involved is then completed.