I'm considering developing an app (iOS) my customers can use to send me files (typically PDF but could be other types, couple of MB at most). Sending the file has to be simple, just select and click Send. There are may ways to send/submit the file, e.g. (in no particular order): - E-mail - Dropbox (or similar) - FTP - Web service - Some horrible custom client/server contraption The files aren't top secret stuff and it's not the end of the world if the sending fails but but reliability is more important than security and simplicity (on both ends) is a must. I need to be able to group the files I receive by sender. Any thoughts on what's the best approach?

X.4nn are ISO standards that describe mail systems at various levels. I assume ATLAS 400 is a product/service that supports X.400 messaging and then the question becomes how do you interface with it. Back in the day the X.400 systems I worked with (ISOCOR, Exchange) had file gateways, you simply prepared a valid X.400 binary object and that was picked up by the mail server. ATLAS 400 may have other options, their website is in French so I can't tell. If you need to generate X.400 objects then you'll see the standard is complicated, like all international standards, but since you're presumably only sending, and fairly simple e-mails at that, you only need to cover a limited subset. If you can't find any libraries that you can use I may be able to dig up some old code (25 years+, plain C) that might be of help. Writing it from scratch isn't terrible if you're used to reading standards, BNF etc. I assume the EDI part is a solved problem. You probably don't want to write your own code for that.