IMAP not responding

Hello everybody,
above all, I would like to take my hat off for this magnificent work and this magnificent community.
I was able to create websites thanks to virtualmin (wonderful experience)
my problem is, and it does not seem too complicated for an expert, when I create an email for a virtual server and I connect with usermin everything works fine I can send emails and receive them without problems however if I want to configure it on a smartphone I have the error that the “imap is not responding”
Thanks for your help!