Hi All ,

In my current environment we have one exchange forest with exchange 2013 and another exchange forest with exchange 2010.

Lets say a mailbox user from exchange 2013 forest is sending an mail along with attachment of (1.7 MB) to one of the mailbox user in same exchange 2013 forest and also to one of the mailbox user in exchange 2010 forest .

So on such case the size of the delivered message to the both the mailbox users in the different forest varies.We have found that with the help of message tracking logs in both the forest.

 I mean the mailbox user in the same forest (i.e exchange 2013) receives an message with a size around 1.7 MB and the mailbox user in the other exchange forest (i.e exchange 2010) receives an message with the a size around 2.25 MB .

Questions : 

We would like to know that how the size of the delivered message varies between the mailbox users in the different exchange forest ?

Is this is the default mail delivery behavior in exchange between two exchange forest ?

Note : On my end i thought that the message from the exchange 2013 forest to exchange 2010 forest was processed two times by the categorizer (one time in exchange 2013 and one time in exchange 2010) because of that size is varied .But i am not sure on my end .

All of your suggestions are much appreciated .

Hi S.Nithyanandham,

As you pointed out the Categorizer, some more info for you to dig in.

I feel because your email is MIME encoded - the binary data is changed from 8-bit data to (normally) 7-bit data, in order to be reliably transmitted between servers. There are a few different encoding schemes, but they will typically add a 33% overhead to the size of your email.

So for your 1.7MB of email

33% of 1.7 = 0.561MB

Is actually 1.7+0.561 = 2.261MB of data post conversion.

NOTE:- Exchange to Exchange internel even though it uses SMTP is 8-bit, which only Exchange servers would understand. Once it moves out of the Org its been converted to 7-bit adding the 33% overhead.

Types of message size limits:

The size of the message can change because of content conversion, encoding, and agent processing.

Message conversion for external recipients   This type of content conversion includes the Transport Neutral Encapsulation Format (TNEF) conversion options and message encoding options for external recipients. Messages sent to recipients inside the Exchange organization don't require this type of content conversion. This type of content conversion is handled by the categorizer in the Transport service on Mailbox server. Categorization on each message happens after a newly arrived message is put in the Submission queue. In addition to recipient resolution and routing resolution, content conversion is performed on the message before the message is put in a delivery queue. If a single message contains multiple recipients, the categorizer determines the appropriate encoding for each message recipient. Content conversion tracing doesn't capture any content conversion failures that the categorizer encounters as it converts messages sent to external recipients.

Understanding e-mail attachments:

When Exchange sends an e-mail to another host via SMTP, the message size may change due to the encoding used to package it. This is doubly true for messages with attachments, since the only way to send e-mail attachments is to convert them from plain ASCII to MIME or UU-encoding the message.

References:

SMTP has more overhead?

SMTP Protocol Extensions for Exchange Server 2007

Why real e-mail size is higher than my attachament and text size?

This page has a good explanation of Base64 encoding, usually used when sending emails:

The number of output bytes per input byte is approximately 4 / 3 (33% overhead) and converges to that value for a large number of bytes. More specifically, given an input of n bytes, the output will be 4[n/3] bytes long, including padding characters.

Hi Nithyan,
There is difference between Exchange 2010 mailboxdb architecture 2013 mailboxdb architecture.
Please check my comment in the belwo post.
https://social.technet.microsoft.com/Forums/office/en-US/bc137303-e4d7-4acb-b804-018f1b362879/should-i-have-limits-on-mailbox-sizes?forum=exchangesvradmin

Hi,

From your description, I would like to clarify the following thing:

Base64 encoding increases the size of the message by approximately 33%, so the values you specify for any message size limits are approximately 33% larger than the actual usable message sizes.

Hope this can be helpful to you.

Best regards,