Are you emails being rejected by a particular set of domains? If yes, given below are the steps that you’ll need to take to get this issue fixed quickly.
How can I check the above?
1. Try sending mails from your account to any other email account you have.
2. Check whether the mails are sending/receiving.
How can I solve it?
1. If you are able to send/receive mails to some of the email accounts, (eg: you are able to send to email@example.com and not able to firstname.lastname@example.org) then the issue might be because your account has been blocked by that server. Contact the hosting provider of that account to which you are having the issue.
[Note: Try checking the above steps directly from your webmail. Please donot use the mail client for checking because it can be the wrong configuration of the mail client which may also lead to non functioning of mail.]