when i use eMClient in the same LAN where my back-end MS Exchange resides, i see no delays in opening attachments. But if access eMClient over the WAN attachment opening times are extremely slow to the point that it is not usable. It appears that just because you have the mail downloaded the attachments are now fully downloaded. when i hover over attachments in an email, i can see if it is downloaded or not and if it is “not downloaded”, then trying to click over WAN is painfully slow. Any ideas please?
Hi Ajith,
Of course it will take (a lot of) time when you try to open an attachment which has not been downloaded yet.
You can force e-mails to be downloaded for offline use, but strangely, I can not quickly find that option now. 
Hi Ajith, as Hans mentioned you can use the “Download messages for offline use” option for your account, that way all your emails will be downloaded automatically and will be available even when use eM Client over WAN.
Just go to Tools > Accounts > Your account > Mail > Download messages for offline use.
This only works with IMAP and POP accounts, if you’re using Exchange, emails should be downloaded automatically, but not attachments. To download attachments automatically right click your account in the left sidebar and select Properties > Offline, and check the option to download attachments automatically.
Hope this helps,
Paul.