Receiving email, but no emails will send!

My Roundcube email (my least favorite part of Homestead) is receiving emails, but nothing I compose will send. I hit the send button and get the little gray box that says it sent, but the receiving party doesn't get the email. I've been having this problem for some time. I'm sending to both domain name emails and gmail accounts, my emails are not getting through and spam & trash folders have been checked - what do I do?? Sorta makes the entire thing useless!