Automated migration of mail content only to existing accounts in Plesk will take place if the following conditions are met:
migrmng.exe.config file has no e-mail account information. (For more information, consult "When Not to Use Migration Configuration File" and "Using Configuration File to Specify E-Mail Accounts for Migration" sections).
To enforce migration of mail content only to existing accounts in Plesk manually, specify the mail transfer protocol (POP3 or IMAP4) and do not include any e-mail account information in the migrmng.exe.config file. For description of the file and instructions how to use it to specify e-mail accounts to be migrated, consult the "Using Configuration FIle to Specify E-Mail Accounts for Migration" section.