I evaluated the Demo service project where I successfully connected my custom email instead of the default one. After deciding to go with this product, I deleted the Demo service project and created a new service project, where I also wanted to connect the same custom email I had, but the system does not allow this.
I went to Project Settings > Email Requests > Connect a custom email account > Other > Enter email and password > Next > enter mail server, protocol and port > Save button is disabled.
Thanks a lot. Actually, I cannot switch to another email. I already raised the ticket to Atlassian support.