This week Microsoft made some changes on their mail servers, which disabled OAuth login for the SMTP protocol on their accounts. This means those using free MS accounts, such as @outlook.com, @hotmail.com, @live.com and similar stopped being able to send emails from eM Client, the messages either got stuck in Outbox or you received this error:
[SMTP] MailClient.Accounts.AuthenticationAbortedException: Authentication aborted.
We have reported the problem to Microsoft and also created a workaround for eM Client which you can use until Microsoft fixes this issue on their side. Please follow the link below to our Knowledgebase:
eM Client: Cannot send emails for Outlook.com accounts - Authentication aborted