What could be causing a sudden increase in the bounce rate on Aweber?

Has anyone else experienced a similar issue recently? I usually delete all my undelivered emails every few days, but the last two emails I sent had a bounce rate of 3%, which is unusual for me. I don’t think it’s fake traffic, as I didn’t have many new leads the day before. The majority of the emails were from Hotmail, with the remainder from MSN or Live.com. I have checked my domain with mxtoolbox.com and it doesn’t appear to be blacklisted. I still have relatively good open rates with these emails. Does anyone have any ideas on what the problem might be?