I've recently set up a new laptop with Outlook 2013. Over the last few days there have been some (but not all) people who have complained about e-mails I've sent from that machine. They will receive mail when I send it but the same mail will reappear unread the next day for some people (and I can only see the mail in the Sent-folder once). For example, I've sent an email two days ago at 22:34 and it has reappeared yesterday and today (with the correct timestamp) as a new mail.
This is a basic POP3-account that keeps mail on the server and it is a shared address for an organisation I volunteer for. There is one other person who reads the mails as well. I copied the .pst from my old computer as it contains an archive of email over 3 years old and older mails are no longer on the server. I've set up the account with the same options I used for previous computers and there is nothing odd or specific about the configuration.
I send mail on multiple accounts from that laptop and there is only one account affected. (they are on different hosts so this might be related to the host and not my client) Some of the mails I send are also sent to another one of my addresses and I only receive that e-mail once. Sometimes I CC the account from which I send to keep the other person using the account in the loop and he will receive the mail twice, whereas everything seems normal for me and I will only receive it once. (Which is very confusing...)
I'm pretty much at a loss what could cause this. Since it only happens when I send I suspect my own mailclient to be at fault but there is nothing odd about the configuration.