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I host my own email server courtesy of Synology MailPlus Server. In an effort to increase security I created a username consisting of 25 random characters. I monitor the mail logs of this server daily and to my surprise I can see a small number of logins from various IP addresses failing to authenticate using this username. I have configured the mailbox for IMAP using SSL/TLS and prohibit plaintext authentication over unencrypted connection. I'm at a loss how this username is becoming visible to others?

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    Do you by any chance use Outlook to access your mail via IMAP? Are these IP addresses from Microsoft? See security.stackexchange.com/questions/209163/… Feb 2, 2023 at 16:19
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    By "username" do you mean the email address user name or the admin account username? If you mean the email address, that's considered public data and is fairly easy to discover.
    – schroeder
    Feb 3, 2023 at 9:03
  • I purposely did not use an email address as the username for this email account but instead chose a random 25 character username. This username is not the admin account username.
    – davewash
    Feb 3, 2023 at 18:24
  • No I do not use Outlook to access my Mail server, my email client is Apple Mail and no the IP addresses do not belong to Microsoft.
    – davewash
    Feb 3, 2023 at 18:35

1 Answer 1

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The email address is visible:

  1. to the recipients
  2. in log files of SMTP servers transferring email from and to it.

Feel free to guess where the leak happened you clearly state the above options are excluded in your case.

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  • I appreciate the email address will be visible to others but I did not use the email address as the username for this mailbox but generated a random 25 character username instead. My question is how someone got hold of this username?
    – davewash
    Feb 3, 2023 at 18:28

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