Author: timbarrett

From the Help Desk file…

A client running SBS is having problems with large attachments bouncing.  They are using SMTP, not POP3:

• Each month they get a a particular inbound email from an external sender w/ a sizeable attachment
• Last month the attachment was 7,403 KB and it was delivered just fine
• This month the attachment was 7,562 KB and it was rejected

Error Message:

“The following recipient(s) could not be reached: This message is larger than the current system limit or the recipient’s mailbox is full. Create a shorter message body or remove attachments and try sending it again.”

Gut Reaction:

The ultimate computer troubleshooting question – “What changed?” 

The attachment is bigger (but not much).  And it’s likely that the users mailbox has grown in the last month, right?  Well, not so fast.

Server Settings:

• Mailbox Store Storage Limits:  Warn @ 175 MB / Prohibit @ 200 MB
• Client mailbox size:  22 MB
• Default SMTP Virtual Server Properties: Limit message to size (KB) is unchecked
• Exchange Delivery Defaults:  10,240 KB send / 10,240 KB receive
• User Delivery Defaults:  10,240 KB send / 10,240 KB receive

So the attachment is well below the 10 MB limit, and the user has plenty of storage space.  What gives?

The Smoking Gun:

I did some research and found the following KB Article:

“How to set size limits for messages in Exchange Server”
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.”

Yeah – you read that right, 30% overhead for SMTP email attachments. 

Let’s do the math – a 7,562 KB attachment with a 30% increase (7,562 / 0.7) = 10,802 KB.  And that’s larger than 10,240 KB folks!  I talked to Vlad for validation, and after the obligatory mocking session, he confirmed that’s about right for the overhead.  He also said something about an 8-bit attachment going through a 7-bit system, but that’s over my head.

Resolution:

I bumped the limits up to 15,360 KB, so they should be able to receive a 10,752 KB attachment (a true 10 MB), sent another test email, and it worked!

So here’s a rough conversion chart of what your settings need to be to get ‘true’ attachment sizes through the server:

Physical Attachment / Actual Height Needed

• 5 MB / 7,200 KB
• 6 MB / 8,600 KB
• 7 MB / 10,00 KB
• 8 MB / 11,400 KB
• 9 MB /12,900 KB
• 10 MB /14,300 KB
• 11 MB /15,700 KB
• 12 MB /17,100 KB
• 13 MB /18,600 KB
• 14 MB /20,000 KB
• 15 MB /21,400 KB

To plug in that setting, just navigate to:

Server Management / Advanced Management /  Exchange / Global Setting / Message Delivery / Properties / Defaults tab:

I hope that helps somebody.  And thanks Vlad, my favorite Exchange MVP 🙂