Error 'This is a permanent error' on undeliverable email via Add Email option

SCENARIO 1

“A message that you sent could not be delivered to one or more of its recipients. This is a permanent error. The following address (es) failed: 
>>> {Your Email to Salesforce Address} (Undelivered): 554 Transaction failed: {Email Address you used to add the Email}, (This is the email address that is missing or incorrect in your settings) is not authorized to send emails to this service."

OR

"A message that you sent could not be delivered to one or more of its recipients. This is a permanent error. The following address(es) failed

>>>{Your Email to Salesforce Address} (Undelivered): 551 SenderAuthorization: {Email Address you used to add the Email} is not authorized to send emails to this service.

1. Click Your Name | My Settings | Email | My Email to Salesforce
2. In the 'My Acceptable Email Addresses' field, add the email address(es) that you will be using to add emails to Salesforce.
3. Click on Save.

1. Click your Avatar | Settings | Email | My Email to Salesforce
2. In the 'My Acceptable Email Addresses' field, add the email address(es) that you will be using to add emails to Salesforce.
3. Click on Save.

SCENARIO 2: 

“A message that you sent could not be delivered to one or more of its recipients. This is a permanent error. The following address (es) failed:

>>>{Your Email to Salesforce Address} The recipient's mailbox is full and can't accept messages now. Please try resending this message later, or contact the recipient directly.
{Your Email-to-Salesforce Address}  #5.2.2 SMTP; 552 Message is too large; 10485760 bytes max> #SMTP#"

This error message is related to the size of the files attached to the email. The size limit for an attached file is 5 MB when attached directly to the related list. The size limit for all files attached to an email is 10 MB.

SCENARIO 3: 

"A message that you sent could not be delivered to one or more of its recipients. This is a permanent error. The following address (es) failed:

>>>{Your Email to Salesforce Address} (Undelivered): 554 System.LimitException: Regex too complicated (System Code)"

When adding an email to Salesforce using Email to Salesforce or Salesforce for Outlook, the record will be added as a task. The email body will become the task description and this field has a limit of about 32000 characters. If the email body is too large, the message will be rejected.

You can contact Support for increasing the 32000 character limit.

SCENARIO 4:

A bounce back message is being received if there's a universally required field on Activities.

"The apex class Messaging.EmailToSalesforceHandler failed due to: System.UnexpectedException: Required fields are missing: [fields]"