It's possible that it was caught by a spam filter on her email program instead of by the server. Not much to do about this other than check DKIM/SPF and ask her to whitelist you.
Josh is right. If your client adds your email as a safe sender it will not be block but the email client.
You can also test sending a TEXT email instead of HTML, Some email clients mark as spam emails with a lot of images or links to other domains that are not the senders domain.
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Right, if it's being accepted by their server but not actually delivered to their inbox, or if it's going to their spam folder, your best course is check that SPF and DKIM are set up properly (you can check DKIM by looking for "dkim=pass" in the headers of a test email).
You might also consider running some smart lists to see if you've been sending to a lot of leads who are regularly bouncing, or who have been in your database for a long time but never taken any action (never clicked or opened emails, visited pages, filled out forms). If you mail to a lot of nonresponsive leads, it will damage your domain's reputation as a sender, making your emails more likely to get blocked or sent to the spam folder.
Clearing out such leads is also a best practice to avoid hittings spam traps.
Thanks for all the help and advice!