Last week I looked at a few potential pitfalls to watch-out for when installing Snow Leopard and asked you to report any new ones you'd discovered. Overwhelmingly, you identified Apple Mail as the main causality with many users reporting they either can't send anything (some receiving the message "Error 54: Connection Reset By Peer") or that it simply crashes on opening. How on earth Apple didn't test this is not clear but as some of us at Softonic think, maybe they just presumed only dinosaurs use a Mail client nowadays ;)
As regards sending problems, it may not be Apple's fault however. It seems that it could be related to a SMTP issues specific to different ISPs. Obviously, there's no way that Apple could have tested Mail with every ISP in the world beforehand. One Apple forum user explains:
With some ISP, notably those that are cable, the SMTP is not separately authenticated, but rather is trusted based upon the IP address of the Modem they provide you. For such ISPs and SMTP, the authentication must be set to None, and no password or username enter for the Outgoing Server. It is simple for Mail to guess wrong about this, and you must set it right in the settings for the SMTP.
However, other uses have reported that changing SMTP settings still hasn't worked for them. Here's a collection of suggestions to help you get Mail working again:
Update SMTP Authentication
This is Apple's only official response on issues related to sending problems. Go to the "Mail" menu and select "Preferences". Click "Accounts" and then select the account the one having problems. From the SMTP server pop-up menu at the bottom choose "Edit SMTP Server List". Click "Advanced" and then select the "Authentication" pull down menu and select the appropriate authentication type (from your ISP or the Mail Online Setup Assistant). Fill in the username and password and click OK.
Edit Outgoing Mail Server
Go to "Mail" and "Preferences" and select "Edit Outgoing Mail Server". Select "Advanced" and then change the radio button to Custom Port 25 (depending upon your ISP). Se the Authentication to "None" and click OK.
Move Mail folder to desktop
Sounds simple but apparently it's worked for a number of users. Locate your mail folder in "/username/libary" and then move it the desktop. You can then import your mail messages by going to "File" and then "Import" and browse to the Mail folder on your desktop. You may find your mail is a bit disorganized after importing but you'll just have to put everything back in it's right place and folder manually.
Ideally this should be done just before and after your upgrade to Snow Leopard. If you've already upgraded however, you'll just have to use Disk Utility to repair them afterwards.
Use Time Machine
If all of the above fails, then just use Time Machine to rollback to reliable old Leopard.