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
“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.”
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.
- 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”
“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.
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 🙂