Apple's MobileMe service problems seem to be continuing as a number of users have reported having authentication problems in which they're prompted to enter their user name and password each time a message is sent. According to MacFixIt, the credentials sent to the server are then rejected.

One user on the relevant Apple Discussions board who went by the handle GaryJ73167 offered the following description:

"I enter my password, I can then receive my email; but, only moments later, it keeps asking me for my password. I keep clicking on "remember this password in my keychain", but, it makes no difference."

The problem doesn't seem to be ISP-specific and users from assorted providers as well as several foreign companies have reported the issue. Still, Comcast users seem to be the most affected and users with multiple e-mail accounts have noted that one service will fail in its task while others won't, a sign which indicates that Apple's Mail.app program is running correctly and that the server is leading to the error.

To date, users have tried the following fixes with varying degrees of success:

Port changing: Change the outgoing SMTP server port settings Most ISPs use either port 25 or port 587 as the outgoing port number, so while by default Mail may put port 25 as the outgoing port number, users might try changing this to 587 to see if that helps. To get to the outgoing (SMTP) server settings, go to Mail's preferences and select the affected account. Then in the "Account Information" tab select the outgoing mail server menu and choose "Edit server list". In the resulting window select the SMTP server for the ISP and under the "Advanced" tab there should be options to set the port, have the option to use SSL, and change the authentication settings.

Change the incoming server port settings: To get to the incoming mail server settings, go to Mail's preferences, select the problematic account, and click the "Advanced" tab. As with the outgoing servers, incoming servers also have various ports that are used, and like port 25 on the SMTP server, the old default for incoming mail servers is port 110 (for POP) or 143 (for IMAP). However, ISPs may also use port 993 or 995 in this setting if SSL is used (similar to port 587 for outgoing mail).

Change the authentication method: For both incoming and outgoing mail servers, users can change the authentication type to see if the ISP will accept it. The default and most commonly used method is "Password", but users have had success with "MD5 challenge-response" and may also have success with NTLM authentication. If an authentication method is not supported, users will get an error message that clearly states lack of support for the given method.

Remove MobileMe synchronization of Mail preferences and account settings: Since this problem seems to occur for users who installed the MobileMe updates, it may be possible that Mail account synchronization is to blame, especially since some users get intermittent success with account settings changes. To at least help in the troubleshooting of this problem, users might choose to disable synchronization of all Mail information (accounts, rules, signatures, etc) in the MobileMe system preferences.

Other users have tried removing or disabling account from Mail except for the affected one, then troubleshooting that account.

A final suggested fix for the issue is to reapply the MobileMe updates, which may help fix the problems. Users can find the updates in the "Security Updates" section of the Apple Downloads page. It's recommended that upon downloading the updates, boot into "Safe Mode" to apply the update after fixing permissions with the Disk Utility program. The MobileMe updates are not currently available as standalone downloads.

If you've seen these issues on your end or have come up with a fix of your own, let us know over in the comments or forums.