Because soon those email services just won't let you - and for good reason.
To prevent address spoofing by spammers, some of the major webmail services (including Gmail, Yahoo and AOL) will reject and bounce back any email with a "From" address of theirs which isn't being sent by themselves.
This is why you shouldn't use Gmail, Yahoo or AOL senders in E-goi, as soon virtually none of your emails won't be delivered! We've blogged further about this (it's a Yahoo-centric post, but Gmail and AOL do pretty much the same thing).
Try to use an address hosted in your own domain instead (eg. me@mycompanyname.com and not me@gmail.com). A registered domain is not only suited for email marketing but can also be manually set up for the highest possible deliverability.