sent emails from another computer don't show up in the designated folder

Hi Guys, I use two email accounts in my thunderbird, one of them is being used by me and a colleague of mine who reads and writes emails through this acount but from a different computer. For some reason, I can't see the emails that he has sent from that account, only the emails I send from my computer are saved in the sent folder. Seems like Thunderbird only updates the inbox but not the sent folder. I also use an add-on that organises messages in threads - the sent messages of my colleague are also absent from the threads. The only way I can read them is through the quoted text at the bottom of a replied email. What can I do ? Thanks a lot in advance !