some emails cannot be sent successfully in outlook,why?

[Gavin]

For most of the emails, i can send successfully. But some certain emails i
cannot send normally. Why? And this problem happened for three days, but in
the past several months i can do it. Some special setting needed, or sth
else? THANKS!

 

[Roady [MVP]]

Hard to say what is going on without any details. Telling us what happens
when you try would be helpful as well. Any error messages perhaps as well?