1.10 is very end of life. I would recommend that you upgrade to something contemporary.
Such as the current stable 1.15.2. It will very likely fix your issue.
As a side note, over the years we have use Outlook XP, 2007, 2010, 2013, 2016, and 2019 desktop apps with osTicket and have not had this issue before. We did upgrade osTicket as newer versions came out though so chances are you have run amok a lack of backwards compatibility or one of the many email improvements that came out in 1.11.x, 1.12.x, 1.14.x, and 1.15.x