The most common cause is that you're using token values in the "From" or "Reply-To" field that don't have a valid default value set. Tokens can only use the default values when sending as a sample (unless you specify a lead record for it to sample off of) and if it can't find a valid default value, it fails trying to leave the mail server. Past that, it could be a bouncing or spam folder issue. Support could get a better grasp on what's going on by looking at your instance.
Had the same issue on a co-workers email program, double checked after seeing your reply and saw the default value was missing.
Thanks for the reminder!
Another reason this could happen is you may not have a valid unsubscribe link in the footer. We use snippets for our footers and sometimes forget to insert the appropriate country snippet when doing our testing - resulting in no email being received. I wish there was some warning/notification that Marketo would provide prior to sending emails without a valid unsubscribe link (for those that are non-operational).