If the mail server supports the IMAP protocol, you can use a library such as MailKit to access the "Sent" folder, but whether the message(s) you expect to be there are there or not will all depend on whether the client that sent the message(s) also appended them (via IMAP or otherwise) to the "Sent" foolder.
FOr example, if you are sending mail using System.Net.Mail.SmtpClient, then any message(s) you send with that API will NOT end up in the account's "Sent" folder unless you explicitly append the message to that folder using a library such as MailKit to do it.
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