Depends on your definition of large.
I find sending large attachments via email to be well, wrong to be mild about it.
What I normally do is place the actual file on a web server.
Give the link to the file in the email. The recipient can then click the link and download the large file. So I would email something like
http://my.domain.com/emailAttachments/theBudget.xls
Now, Zimbra has a feature where the admin can set the file size for attachments as well as for files uploaded to the Briefcase. So, if you want to provide access to large files exclusively via zimbra I suggest 2 things:
1. have your admin set the size right for you
2. define a briefcase and share with whomever you want by granting access
If you do that you allow them to upload and download. In email just tell them what file to grab. Avoid, clogging up the email server with large files. It's and email server and folks shouldn't use an email server as a file server.
Good luck. Zimbra rocks...that thing they produce is Redmond, well.....nighmare
peace