Problems with outgoing emails

Hi, I have been away for a month and upon my return, manager will not send emails out.
It comes up with:
“The remote certificate was rejected by the provided RemoteCertificateValidationCallback”,
My Test Emails are giving me the same message.
Could you please help.
Many thanks in advance.
Brenda

I guess that you use gmail and you need to follow carefully the detailed instructions to deal with 2FA at Google Gmail | Manager if not using gmail but MS Office 365 or Yahoo email check Troubleshoot email issues | Manager