iPhone Email Applications and Mail Add-Ons
Email 'n Walk - iPhone Email Application
Email 'n Walk lets you see what's behind your iPhone while you are composing an email.
GPush - Push Gmail iPhone App
GPush brings push alerts for new Gmail messages to your iPhone or iPod touch.
mBox Mail - Windows Live Hotmail Email Program for iPhone, iPod touch
mBox Mail works much like the iPhone's built-in Mail but talks to Windows Live Hotmail accounts and integrates your Windows Live contacts with your iPhone's, too.
PushMail - Push Email iPhone App
PushMail alerts you using iPhone sound and pop-up messages to new mail sent or forwarded to a personal PushMail address.
ReplyButler - iPhone Mail Add-On
ReplyButler makes it easy to send standard replies and oft-used text with the help of quickly inserted message templates.
Syphir - Advanced Gmail Rules and iPhone Push Service
Syphir adds some smart filtering options to Gmail — figuring the likelihood you will need to reply, for instance, or a message's age — and can take smart action, too, such as delaying mail, alerting your iPhone or applying labels.
iPhone Mail - Mobile Email Program
iPhone Mail does a great job rendering emails and attachments pretty, swift and useful.