What is the best application to Backup Outlook 2015 for Mac Email Database automatically?

This is a sign of corrupt Outlook Mac database. This can be resolved by rebuilding the database. Try out these steps:

1.Quit all applications
2.Determine the size of your identity
3.Backup Current identity
4.Rebuild the database
5.Create new identity

In case if this does not resolve your problems then you can fix this issue by using third party software like Stellar Phoenix OLM Repair. I suggest you to try its demo version first: https://www.stellarinfo.com/email-repair/outlook-mac-repair.php

Hi Scott
I am a Mac User also.
Ran my own Mac repair business for 18 years...
I remember this particular problem with Entourage/Outlook for Mac
I stopped using Outlook & Office years ago because it goes corrupt when too many emails are stored in the email data file. About the 1gb file size. A well known problem. I use Apple Mail these days. Note..It will Import emails from Entourage.
All this data is kept in the Microsoft User Data folder inside Documents Folder... if my memory is correct.
Make a backup copy of this folder if you can... before fiddling.
I used a Microsoft Utility Program called "Microsoft Database Utility" to fix and clean up the Email data file. Then Entourage opened up the email again and I got rid of excessive emails containing photos.
I don't even have MS Office installed on this Mac any more but I have located "Microsoft Database Utility" program which is about 2.3mb in size. It's probably inside your MS Office / Office folder already. Use Apple Spotlight to search or... a free program called EasyFind works great.
Open the program and point it to the Microsoft User Data folder where Entourage email data file is located. To locate the actual email data file...Use "Cmd I" to find size of suspect file. The email data file is a Fairly big one.
I am using a 2007 MacBook Pro running OSX 10.9 Maverics with the free "Pages" for reading Word Doc's & "Numbers" for Excel SS and it all works seamlessly. Dont have a need or want to pay for MS Office anymore as it never got many updates anyhow.

Hope this Fixes your Problem
Cheers
Trevor

On Side Subject.... Depending on age of your Mac...You can get Maverics OSX10.9 for free. Download the flu .DMG file. Use CCC (Carbon Copy Cloner from Bombich Software) to do a full Clone Backup of your current Mac HD to another external hard drive before installing new clean OSX 10.9 system then you get asked to Migrate your old system data back and you end up with a new Mac and all your data in place. Get free "MacTracker" to get your specific Mac Specs & what max OS it will go up to.

Reinstall software. If this is helpful, please post as such. TY