I find that mails come in at random times of the day, which kind of makes sense in a way.

Due to the way some anti-spam products work, companies are forced to send mails in "bursts" with a hundred recipients or less at a time, then wait some time, then send some more and on it goes.

If they send their entire customer base (could be a hundred thousand customers) in one go, some anti-spam products check the email type and category, and may place a stamp of it in a possible spammer category, which then gets replicated over to other databases maintained for this purpose, and possibly flag the company as a spammer.

Very annoying for IT people to work around, of course

Maybe one is just very unlucky and always ends up being one of the last to receive the mail due to their email address name (possibly the mails are send in alphabetical order of recipient) or some other rule...