Larger attachments won't download on iOS.

We use about 10 iPhones, all with iOS 13.x.x. Our mailserver is running MDaemon 19.5.1 and ActiveSync 14.1. Since a few months we got problems downloading larger attachments on these iPhones. For example, a JPG image of 60 kB is not a problem, but when it is 4 MB it just won't download. When the email is opened on the phone, grey icons show up with a "download" link. Tapping this icon changes it into "downloading", but nothing happens. Can't find any working solution on the www. Tried reinstalling account, reinstalling app, reboots, network reset, etc. No glory. Can't find anything particular in the log files. I am not sure if it is MDaemon related but all accounts are setup as Exchange ActiveSync. Different IMAP accounts from other providers on the same phone do not show these problems. All attachments do download well in that case.

And, no problems on Android devices.

Has anyone seen this behavior?

Best regards,

Pim