Symptoms

You may be presented with the following error message in Outlook 2016 when using a Gmail / Google Apps account:

[ALERT] Message too large. http://support.google.com/mail/bin/answer.py?answer=8770

Reason

Google Mail has a maximum message size including attachments of 25MB

Troubleshooting

You have already tried to delete the offending email that failed to send however periodically you still receive the same error message above

Resolution

You need to delete any trace of the offending email in any folders that sync with Google.  This includes Deleted Items, All Mail etc.. The easiest way to find all mail larger than 25MB is to set up a search folder in Outlook.  Here’s how to set up a search folder in Outlook 2016 in order to show any emails in ALL folders which are 25MB or larger

1. In the left hand sidebar where all your folders are displayed, scroll down until you see Search Folders and expand it
2. Right Click on Search Folders and choose New Search Folder
3. Scroll Down to Organizing Mail and choose Large Mail, under Customize Search Folder click on the Choose button and type in 25000, Click on OK (see image below)
   
4. You will now see a Large Mail folder in the Search Folders pane and can quickly locate any messages over 25MB