For organizations that need to send very large attachments there can be several issues that occur. One item that came up recently that is not spelled out for Office 365 seems to be a behavior many users experienced with Exchange 2000/2003 and routing groups. With Office 365 most organizations currently configure a Hybrid environment. Recently I experienced emails being denied when sending a large file between a user hosted on Exchange Online and another hosted on-premises. Below is a rejected email with a 24 MB attachment from an Office 365 user to a Hybrid Exchange on-premises user.
The same attachment could be sent to other Office 365 users and when viewed in Outlook was reported as being 24 MB in size.
What seems to be happening is the occurrence experienced by administrators of Exchange 2000/2003. Listed in an older article pertaining to Exchange 2000 and 2003, halfway down the page in User mailbox settings there is a small but very important note. http://support.microsoft.com/kb/322679
“Note The size of SMTP messages that are sent between routing groups and to the Internet increase by about 30 percent if they contain binary attachments or other 8-bit data.”
Relating this back to the current issue, 32 MB is about 30% larger than 24 MB.
Why is this happening? Some systems when sending files do not allow all data values, like binary, and translate these files into plain text expressions before being sent. This codec process adds the approximate size increase of 30%. Sounds familiar.
What types of attachments are affected? Pretty much every file type except text files. In other words, pretty much every file type.
What this means is when sending “Binary or 8-bit data attachments” between your Hybrid deployment of Office 365, the actual file size attachments are currently limited to about 19 MB. Even if the limits are increased in the future, this file inflation will still occur. Additionally this means most every email with an attachment sent outside of Office 365 is actually 30% larger. Many companies leave the default 10 MB message size for the on-premises Exchange environment and you may start seeing issues between intra-organizational emails in hybrid deployments when attachments are larger than 7.69 MB.
While this behavior is not an issue with Office 365 and the 25 MB attachment limit is accurate as it applies to text files, unless someone is sending their ASCII Art collection it would be helpful to set expectations on typical file attachments.