[exim] Attachment sizes - confusion

Hi,

I have been playing with message size adjustments and I have noticed that when setting Exim's message_size_limit = 25M I would expect messages no bigger than 25 MB through.

However I sent myself a file through to test this:
-rw-r--r-- 1 po35 po35 24021574 Feb 15 2015 test-pdf.pdf

Which is about 23MB in size and so was somewhat confused that when it got to Exim to find that I got the following message:
message too big: size=32873715 max=26214400 (25MB)

So whilst I appreciate some growth where the attachment is re-encoded and packaged up to be sent via email I was to be honest mildly surprised that there was a 25% growth in attachment size. I am not going to be at all surprised if someone tells me that this increase can vary dependant on the encoding method and the type of data etc etc - but it does make things a bit of a pain when I need to advertise a hard figure as a maximum size that we will allow through our mail system when that does not correlate to what the user sees as a file size usage on their disk.

Short of picking a number such as 35MB for Exim's benefit and hoping that is enough as I can live with going over the threshold but under is not going to make me popular, does anyone have a better way of calculating what I need to set the limits to to ensure a 25MB attachment can get though (no matter what the encoding etc)?

Cheers

Paul



Paul Osborne
Senior Systems Engineer
Canterbury Christ Church University
Tel: 01227 782751