why does e-mail not reach anyone even though "sent successfully"

I just went through the bad certificate on my mail server routine apparently caused by a virus software update. However, when I send e-mail now, it never reaches anyone I address it to. This started last evening even before the certificate issue was fixed. Every e-mail sent since then has not reached the addressee. I am still receiving e-mail but even replies to e-mail received never get to the addressee. I have more than one e-mail account and even when I send from my main account to a secondary account, it says it is successfully sent, slots it in my sent folder, but it never gets to the recipient. I'd appreciate any help anyone can give me with this. Thanks.