SMTP error in source-build-partners-notifications pipeline

[mthalman]

##[error](NETCORE_ENGINEERING_TELEMETRY=ReleaseStaging-Infra) The SMTP server requires a secure connection or the client was not authenticated. The server response was: 5.7.57 Client not authenticated to send mail. Error: 535 5.7.139 Authentication unsuccessful, SmtpClientAuthentication is disabled for the Mailbox. Visit https://aka.ms/smtp_auth_disabled for more information. [BYAPR05CA0044.namprd05.prod.outlook.com 2026-01-07T00:40:30.380Z 08DE4CF73CFFD718]
      System.Net.Mail.SmtpException: The SMTP server requires a secure connection or the client was not authenticated. The server response was: 5.7.57 Client not authenticated to send mail. Error: 535 5.7.139 Authentication unsuccessful, SmtpClientAuthentication is disabled for the Mailbox. Visit https://aka.ms/smtp_auth_disabled for more information. [BYAPR05CA0044.namprd05.prod.outlook.com 2026-01-07T00:40:30.380Z 08DE4CF73CFFD718]
         at System.Net.Mail.MailCommand.CheckResponse(SmtpStatusCode statusCode, String response)
         at System.Net.Mail.MailCommand.EndSend(IAsyncResult result)
         at System.Net.Mail.SendMailAsyncResult.SendMailFromCompleted(IAsyncResult result)
      --- End of stack trace from previous location ---
         at System.Net.Mail.SendMailAsyncResult.End(IAsyncResult result)
         at System.Net.Mail.SmtpClient.SendMailCallback(IAsyncResult result)
      --- End of stack trace from previous location ---
         at Microsoft.DotNet.ReleaseLib.Helpers.MailSender.SendMailAsyc(String username, String password, String subject, String sendTo, String content, String displayName, String bcc) in /_/src/Microsoft.DotNet.Release/Microsoft.DotNet.ReleaseLib/Helpers/MailSender.cs:line 66
         at Microsoft.DotNet.ReleaseLib.SourceBuild.PartnersNotificationService.SendPartnerEmailAsync(String username, String password, String emailContent, String sendTo, String bcc) in /_/src/Microsoft.DotNet.Release/Microsoft.DotNet.ReleaseLib/SourceBuild/PartnersNotificationService.cs:line 144
         at Microsoft.DotNet.ReleaseCli.Operations.SourceBuild.SendPartnerEmailOperation.ExecuteOperation() in /_/src/Microsoft.DotNet.Release/Microsoft.DotNet.ReleaseCli/src/Operations/SourceBuild/SendPartnerEmailOperation.cs:line 34
         at Microsoft.DotNet.ReleaseCli.Operations.SourceBuild.SourceBuildBaseOperation.ExecuteAsync() in /_/src/Microsoft.DotNet.Release/Microsoft.DotNet.ReleaseCli/src/Operations/SourceBuild/SourceBuildBaseOperation.cs:line 20

Failing run (internal link)

[mthalman]

cc @oleksandr-didyk in case you have any ideas on the cause since you originally implemented this.