Client-specific message size limits – or the reason why iOS won’t sent emails

Last week, a customer complained that he could not send emails with pictures with the native iOS email app. He attached three, four or five pictures to an emails, pushed the send button and instantly an error was displayed.

We checked the different connectors as well as the organizational limit for messages. The test mails were between 10 to 20 MB, and the message size limit was much higher.

geralt / pixabay.com/ Creative Commons CC0

The cross-check with Outlook Web Access indicated, that the issue was not a configured limit on one of the Exchange connectors. Instead, a quick search directed us towards the client-specific message size limits. Especially this statement caught our attention:

For any message size limit, you need to set a value that’s larger than the actual size you want enforced. This accounts for the Base64 encoding of attachments and other binary data. Base64 encoding increases the size of the message by approximately 33%, so the value you specify should be approximately 33% larger than the actual message size you want enforced. For example, if you specify a maximum message size value of 64 MB, you can expect a realistic maximum message size of approximately 48 MB.

The message size limit for Active Sync is 10 MB (Source). This is a server limit which can’t configured using the Exchange Admin Center. Taking the 33% Base64 overhead into account, the message size limit is ~ 6,5 MB. My customer and I were able to proof this assumption. A 10 MB mail stuck in the outbox, a 6 MB mail was sent.

How to change client-specific message size limits?

In this case, my customer and I only changed the Active Sync limit. You can use the commands below to change the limit. This will rise the limit to ~ 67 MB. Without the Base64 overhead, this values allow messages sizes up to 50 MB. You have to run these commands from an administrative CMD.

Make sure that you restart the IIS after the changes. Run iisreset from an administrative CMD.

Please note, that you have to run these commands after you installed an Exchange Server Cumulative Update (CU), because the files, in which the changes are made, will be overwritten by the CU. This statement is from the Microsoft:

Any customized Exchange or Internet Information Server (IIS) settings that you made in Exchange XML application configuration files on the Exchange server (for example, web.config files or the EdgeTransport.exe.config file) will be overwritten when you install an Exchange CU. Be sure save this information so you can easily re-apply the settings after the install. After you install the Exchange CU, you need to re-configure these settings.

The maximum size for a message sent by Exchange Web Services clients is 64 MB, which is much more that the 10 MB for Active Sync. This might explain why customers, that use Outlook for iOS app, might not recognize this issue.

EDIT: Today I found a blog post written by Frank Zöchling in June 2018, which addresses this topic.

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Patrick Terlisten

vcloudnine.de is the personal blog of Patrick Terlisten. Patrick has a strong focus on virtualization & cloud solutions, but also storage, networking, and IT infrastructure in general. He is a fan of Lean Management and agile methods, and practices continuous improvement whereever it is possible.

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