email question - authentication information does not work or are no longer accepted by provider.

- I have not changed anything on my e=mails and suddenly I get this message:   The entered credential or authentication information does not work or are no longer accepted by provider. Please change them.  This is an e-mail I use for my business. I have 30 patients that cannot schedule appointments with me. I lose $1,000 for every day I cannot schedule or confirm appointments. I need help!