Email won't send

I’ve tried hitting reply, and reply-all to different emails… but nothing will send. (Not just to one person… it won’t send to multiple different email addresses). It’s strange because the email will show up in my sent inbox like it has sent but obviously something is wrong because the person isn’t receiving the email. The only way I can send an email successfully is if I hit new email. Any idea what’s wrong? The same thing happens when I try to forward an email, it seems like it sent because it will show up in my sent inbox but the recipient never receives the email. I asked this same question over 2 months ago…and no one from em client has responded.