Office365 - SMPT - Authentication failed - The input is not a valid Base-64 string as it contains a non-base 64 character

Hi Team,

I have several Office365 accounts for work. For one of these accounts I cannot send emails through emClient due to the following error:

[SMTP] MailClient.Accounts.LoginException: L'autenticazione è fallita per il seguente motivo:
             "The input is not a valid Base-64 string as it contains a non-base 64 character, more than two padding characters, or an illegal character among the padding characters.
            at MailClient.Protocols.Smtp.SmtpSendCommand.Authenticate(WorkerStatus status)
            at MailClient.Protocols.Smtp.SmtpSendCommand.Connect(WorkerStatus status)
            at MailClient.Protocols.Smtp.SmtpSendCommand.Execute(WorkerStatus status)
            at MailClient.Commands.Command.Process(WorkerStatus status)
         --- End of stack trace from previous location ---

I have tried to configure another account by adding it as “” account and it works properly. I have already checked Microsoft Graph authorization on Microsoft Azure and this account is authorized for sending e-mails through emClient.

Could you investigate this issue?

Thanks and kind regards,

Can you completely disable your antivirus application and try again.

Hi Gary,

Please note that I configured another email account on the same domain using wizard and it works properly without disabling the antivirus.
I also tried to install emClient on another laptop with different antivirus and the behaviour was the same: one account worked while the other did not.
Finally I tried to disable antivirus but nothing had changed.

Thanks and kind regards,