I think eM client definitely seem to have a problem with attachments bigger than 2.5 Mgb.I have received an email with 3 attachments each between 3-3.7 Mgb. and could not open them even individually. Had absolutely no problem opening them in Outlook or on Ipad. Also the posibility to send the attachnents to the dropbox does not exist on my version. 9.2.1222. Am I missing something?
As a test, can you completely disable any anti-virus, firewall, proxy or VPN, then try again. Sometimes even restarting your router may resolve this. If that doesn’t help, right-click in the message body and choose Properties > Repair > Repair. When that message has been repaired, can you open the attachment?
Go to Menu > Settings > Mail > Attachments and click on the Add button at the very bottom of the window. Click on Custom and choose Dropbox from the provider list.