Mail 4.5 Email Size Limit with Exchange Server?

He said that Apple Mail is notorious for not allowing large outbound emails, though he couldn't say why.

Complete nonsense. The size of any email (with or without attachments) is limited by your email provider; your email client has nothing to do with it. If Comcast says their maximum size is 20 MB, then that's something they impose. But if you're trying to send email from an external mail client and it won't allow it even when it's well below that limit, then it's a Comcast problem.

It's entirely possible that Comcast imposes an even smaller limit on emails sent frm a mail client, but not from their web interface, which use different methods to login, view, send, and receive email than their normal mail servers used by mail clients. You should ask them about that.

But if you're going to send attachments to people, email is a very inefficient method to do it; you're better off using a file sharing service such as YouSendIt.

I have seen this same problem on MS-Exchange with MS-Outlook, on both Macs and with PC's. Our system allows emails up to 50 MB in size, but when something is over 10 or 15 MB, it often (not always) fails. I use primarily MS-Outlook for compatibility reasons, but also use Apple Mail, but I haven't tried a large attachment in a while with Apple Mail. Still, the same problem exists with other Mail programs.

When on Gigabit ethernet, I see a greater chance of a large attachment (> 30 MB) successfully going out. When on wireless, there is a much larger chance it will fail above about 10 MB or so.

And just as you describe, when we log directly into Webmail, we can send attachments right up to the maximum guideline with never a failure.

My theory is that when it takes long enough for the email to physically move out, something changes on the fly that interferes with the check sum or validity of the full package and thus it fails. That would be consistent with the higher internet speeds having more success.

I see this with other internet providers also. Earthlink allows up to 10 MB attachments but when one gets to ~ 4 MB, messages start to bounce, at least where I am. I am not sure if this might be to the actual service provider (your service provider might be leasing internet access through someone else) imposing some other constraint.

I don't think this is a problem with Apple Mail; I think it has to do with the physical and virtual internet providers layering various constraints plus possibly a handshake being dropped when a message takes a very long time to go out.

kurt188 wrote:

He said that Apple Mail is notorious for not allowing large outbound emails, though he couldn't say why.

Complete nonsense. The size of any email (with or without attachments) is limited by your email provider; your email client has nothing to do with it. If Comcast says their maximum size is 20 MB, then that's something they impose. But if you're trying to send email from an external mail client and it won't allow it even when it's well below that limit, then it's a Comcast problem.

It's entirely possible that Comcast imposes an even smaller limit on emails sent frm a mail client, but not from their web interface, which use different methods to login, view, send, and receive email than their normal mail servers used by mail clients. You should ask them about that.

But if you're going to send attachments to people, email is a very inefficient method to do it; you're better off using a file sharing service such as YouSendIt.

I appreciate what you're saying, but I'm seeing the same problem with both Apple Mail and Outlook 2011. I administer an Exchange 2007 server with a mixture of Windows & Mac clients. The Windows clients are using Outlook 2010, the Mac clients are a mixture, some of them are using Outlook 2011, some like myself are using Apple Mail (under OS X Yosemite 10.10). I have seen the attachment-size problem on Macs regardless of client (i.e. Outlook or Apple Mail). We have a 50MB attachment size limit configured on the Exchange server but Apple mail refuses to send anything larger than about 9MB. Ditto with Outlook 2011.

As I'm the sysadmin here I'm in a position to investigate any Exchange settings that anyone wants to suggest to try to resolve this, as the OP was using Comcast so he had to just trust what the Comcast support folks were telling him.

I send 20MB (audio tracks) plus emails daily using Mail.

My provider (ICD) imposes a limit of 300MB, and I have been up to it more than once.

Something you need to be very careful of when talking about attachment sizes, is to distinguish between the size of your attached file(s), and the actual resulting size of the email message. The method used for encoding binary files into ASCII - called Base64 - has a lot of overhead - 33% in fact. Consequently, attaching a 9MB to an email does not just result in an additional 9MB, it results in 12MB (9 * 1.33333). From the Wikipedia page on Base64:

The number of output bytes per input byte is (33% overhead), up to rounding. Specifically, given an input of n bytes, the output will be bytes long, including padding characters.

So a 9MB attachment will add 12MB to the actual size of the email. I also suspect that some email clients are adding even more bloat - I haven't investigated this but given that a typical HTML email message also needs to contain a plaintext version within the same email (divided by MIME-type separators), do both the plaintext and HTML versions each contain the Base64 encoded attachment data? (but even then, I'd need to be sending an 18.7MB file for it to bust 50MB (18.75*1.333333*2) - I'm just trying to send a 9.2MB file).

EDIT: just looked at the raw source of an email I sent with two attachments, one was 679,402 bytes, the other was 928,717 bytes. Total = 1,608,119 bytes. The actual raw source that Apple Mail sent to the upstream mail server was 2,179,777 bytes. The ratio of bloat is about 1:1.35 or 35%. So it looks as though the actual message body of an HTML email is encoded using Base64 (rather than just raw HTML, which is what I've seen some other email clients do), and Apple Mail did not send a separate text-only version of the email inline either (i.e. in its own content-type: plain-text MIME section). They must've dropped that, on the assumption that nobody these days uses non-HTML-capable email clients. Interesting. Unless there's something weird like Unicode encoding resulting in even more bloat during the actual SMTP conversation between Mail and Exchange, then this must be an Exchange configuration issue - there must be a setting not visible in the Exchange Management Console that affects this.

Hi Csound1, thanks for contributing. Is it an Exchange account? Or is it IMAP/POP3?

IMAP.

Current versions of mail can attach items up to 5GB (they send a link instead of the file)

I have worked out a solution for iPhone users with Microsoft Exchange Server and posted it here > Re: Can't Send Attachments Using Exchange Account