lmtp: Trigger autoexpunging only for the last RCPT TO. Otherwise if the autoexpunging takes a long time, the LMTP client could disconnect due to a timeout. The mails would still eventually get delivered though, so it would result in duplicate mails being delivered. An alternative to this would be to keep all the mail_users referenced until the delivery is finished and then autoexpunge all of them at the end. It increases memory usage though and complicates the code, so at least for now it's not implemented.

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