On Zimbra (v. 8.6.0_GA_1153) web client, if you've reached your maximum attachment size, as shown in the message below...
... and you want to adjust the attachment size limit, you need to adjust two things in your Zimbra Admin:
1. Configure > Global Settings > General Information > Maximum size of a file uploaded from the desktop.
2. Configure > Global Settings > Configure > MTA > Messages > Maximum message size.
Don't forget to save your new settings.
