That’s more related to your email service and how you manage junk mail.
That being said, you could take a look at some small things that would help various systems in determining whether to flag the emails sent from the plugin (and WP in general) or not.
First, WP (and therefore WP-Members) does not include “from” headers in the emails sent from wp_mail. You can add that by filtering the headers with wpmem_email_headers.
There are other things you can do to improve your email robustness as well. I do have a post on my personal blog that has some info on this with some links to more detail. Using a real email, adding correct headers, using SMTP; these are all things you can do to improve the likelyhood of the email getting through as well as not being flagged as spam.
