According to this site you are not at the most current version - KB4106: Build numbers and versions of Veeam Backup for Microsoft Office 365

This won’t solve from what I can see in the release notes the speed issue that is throttling on the MS side of things during backups.  You may want to look in to Security Groups where you can add multiple users for backing up things.

This explains how to do it and has a Powershell script to import the Accounts as well - Veeam Backup for Microsoft Office 365 Auxiliary Backup Automation

Also explains where at the bottom picture to manage backup accounts.

Thanks @Chris.Childerhose for the information, in that document I can see that is more when you want to backup Sharepoint and OneDrive, however my backupjob is only for Mail and Archive, do you think that solution applies also for Mail and Archive?

Yes the backup accounts also pertain to Mail & Archive as well because those get throttled by MS too.  I have tested adding multiple accounts and it does help with backup speed but again this is dependent on many factors too as we know. 

Here is a discussion on Reddit about this - (2) VBO365 Slow Backup Speed : Veeam (reddit.com)

@omar.desouza -- considering the bottleneck is the source then it is throttling that is affecting the backups for sure.

I would do two things -

1. Test adding more auxiliary accounts to see if that helps otherwise
2. Contact Support to open a case and see if there are any tweaks that can be done to O365 or the Proxies in your configuration.