My e-mails do not reach the other side
you should describe the problem better to get help.
using a custom mail server?
happens only with one client?
is any error shown?
As sharpdrivetek mentioned, we need more information in order to help you.
Please also let us know if you are using Desktop, Cloud or Server Edition - and if you are not using Cloud Edition, let us know which operating system the computer/server is running.