While using the 'Email Sending Distribution' feature, there could be different reasons for emails not being sent out via the selected multiple email addresses under the 'Send Email From' sequence settings. A few of the reasons have been listed below:
If your email address has been disconnected which may typically happen in a scenario if there has been a password change associated with your email address inside your client provider, or something else has changed regarding your email account you have connected for sending and receiving and thus, this would require reauthentication. You can check out a short video example of re-authenticating your connected email address!
Another scenario would be where your contacts have not been enrolled in the sequence via the sequence 'Enrollment' page and have been enrolled manually under the 'Contacts' page where a different email id was selected under the sender 'From' field while enrolling the contacts manually in the sequence.
Note: The emails selected under sequence 'send email from' settings will follow the same only for contacts enrolled on the sequence page.
To sum it up, if your contacts are enrolled Manually or via SmartList (automatic) under the sequence settings, it will follow the settings for 'sending from' on the sequence settings. If the contacts are enrolled from the Contacts (People/Companies) pages, then it will send from whatever email was chosen in the screenshot shown below, since these are manual enrollments. It will follow the 'send from' that was selected when manually enrolling from the Contacts pages.