Aeschines: Are you running Panther (10.3) or Tiger (10.4)? There’s a big difference between the two. I’m still using Panther, but there are some more powerful features in the new version of the Mail program, including virtual folders.
In Panther, if you want to sort your mail by account, you can set up a rule in the preferences to flag messages by account or move them to a designated folder. If you have things sorted to folders, I suggest you add the account flagging option in your Rule actions to visually alert you to the fact that the message is from a certain account.
If I reply to anything sent to an Inbox, the account associated with it is automatically selected. The key to which account is selected depends on which mailbox you have selected when you create a new mail message, or what account the message you are reading was sent to. If I’m reading, for instance, my Yahoo mail and I want to send a Gmail, I will have to select the Gmail account. Having any other folder than the Inbox selected gives you the first account you created as the default. For me, that was my Yahoo account since I set up Gmail later.
The attachment problem can be solved by going to Edit–>Attachments–>Always Send Windows-Friendly Attachments.
I know that it’s not really helpful to say this about something that is personally annoying, but I’m not really getting why you have a problem with this behavior. It makes sense to reply to mails sent to an account using that same account. If I had a bloody dialog asking which account I wanted to use pop up every time I created a new message, I’d end up using my laptop as a frisbee. It’s always clearly displayed in the message which outgoing account you’ll be using, so if that’s not the account you want, it’s easy to select the one you do want to be using.
For an alternative, I would also suggest Thunderbird. It may have the features you’re looking for. My girlfriend uses it on her XP machine and loves it compared to the other programs she’s used. Bare Bones Software’s Mailsmith is also recommended for power e-mail users, according to MacWorld.
AHunter3: Eudora does have some really powerful features, and I used to use it way back with System 7 through 9. The problem is that even now, when OS X makes it easy to implement multiple languages without hard-coding it into the program and Apple encourages developers to write modular and portable code, it doesn’t support many languages. When I last checked it didn’t even support Unicode properly. While they do have a Japanese version of the program, I don’t want to have to run the Japanese version of the program just so I can read and write mails from Japanese correspondants who only make up about 1/4 to 1/3 of my total mail, especially when Eudora has so many features that even running it in English I would probably get lost from time to time. Those things make it a bad choice for me.