I could this morning, and all of a sudden, I kept getting the dialog that said mail couldn't be sent with selected server. This happens on my Mini and on my MBP. Both 10.8.2. I can send with the same account on my iPad on wifi, and my wife can send on her Macbook Air. What happened?

The Connection Doctor window in the Mail app might give you some clues.

It shows a successful connection.

Reboot your computer.

That didn't fix it. I did have a new outgoing server appear now, however. It has an iCloud identification for my .mac address. That was not there before. I can now send using that as my outgoing server, but that's now what I want. I want to use my Mediacom account for that.

Any other ideas?

Sounds like Mail had a hiccup and chose to use the wrong outgoing mail server for one or more of your accounts.

Go into the account preferences, select each account in-turn and confirm that you have each one set to use the correct outgoing Mail server for that ISP.

For each one, check the box to "use only this server."

I already have those settings in place, and have checked that box.

Quote
iaJim
I already have those settings in place, and have checked that box.

If that were the case then you wouldn't be sending your Mediacom email using iCloud.

...Unless your SMTP settings for Mediacom have been changed.

Choose the "Edit SMTP server list" option from the Outgoing email menu in your Accounts pref's. Make sure that you're using the correct info, including the server-address, username, password and port number.

After making your changes, use the Connection Doctor again to confirm that you're connecting and authenticating on each mail server.

When the dialog box came up, I chose to send the Mediacom email using iCloud to see if that would work. Otherwise, it is still trying to use the Mediacom setting.

I will have to go to the mediacom site to check their settings. Since I didn't change anything knowingly when it stopped, I'm stumped.

Still no luck.

Can you compare the outgoing server settings between your mac and your Wife's MacBook Air to see if there are any differences in the Advanced settings. And also verify that your wife's is actually using that server.

And what sort of setting are you using? Password authentication? I've seen broadband providers block unauthenticated port 25 traffic if they somehow think you're a SPAM source (with very weak / no evidence).

I tried that. Still no good. I guess I'll have to call Mediacom tomorrow and see if they can help. I'm not very optimistic.

One last thing to try: Go into the Keychain Access app and delete the saved passwords for your Mediacom email account (make sure not to delete anything else), then go into the Mail pref's and add the passwords back, both in the Incoming Mail Server setting and in the "Edit Server List" options for the Outgoing Server setting.

Thanks for the suggestions, but it is still not working. I'm wondering what will happen if I delete the account and create a new one. Will I lose my mail from that account?

try tossing this, empty trash and restart.

/Users/name/Library/Preferences/com.apple.mail.plist

although probably a more difficult navigation under ML.

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!#$@@$#!

never do yesterday or today what you can put off til tomorrow or next year.

I found that, but it had "lock file" after it. I deleted it, but it didn't do the trick either.

I created a new account, and didn't delete the old one. I can now send from the new account, but when I put the same settings in the old account, it won't work. Crazy!