If you are receiving Yahoo Error Code MBR #1240 from a mail program such as Microsoft Outlook it may be because you have Yahoo 2 step login verification enabled and need to create a special app password for your mail program.
To create an application password in Yahoo Mail please see this page:
How to create an application password in Yahoo Mail (yahoo keeps asking for my password) | Email Questions
To create an application password in Yahoo Mail please see this page:
How to create an application password in Yahoo Mail (yahoo keeps asking for my password) | Email Questions