iCloud integration (e.g. like Apple Mail Drop)

[JoMailer]

I guess with free cloud storage and expiring links in the eM client, I’m stuck with uploading large attachments in Apple Mail and copying the link to the eM client for now.

Attachment DropBox Link expiration date

This depends on your DropBox account, custom expiration date is not available for their free accounts: In the eM Client trial you have all paid features of the eM Client app enabled.