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I use outlook, and have for about 5+ years. Never had a problem until all of a sudden today I can receive emails, but can't send them anymore. Won't find SMTP servers:

Send test e-mail message: Outlook cannot connect to your outgoing (SMTP) e-mail server. If you continue to receive this message, contact your server administrator or Internet service provider (ISP).

Strange thing is I created the same account on my mac using mac mail, and it works just fine(same account, same password, same smtp settings) it seems the problem is in my Vista 64 PC, but it has worked fine for over a year, and I haven't changed anything that I know of....

How do I diagnose this problem?

EDIT: OK, I can't telnet into it either...I turned off my linksys firewall and the windows firewall...still nothing...ideas?

EDIT 2: OK, seems to be resolved...powered down the router and switched to the backup DSL line...I don't know why it fixed it, but it did...why my mac, on the same network could access will remain a mystery I guess..

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Have you changed your security software recently or done anything else that would cause problems on port 25? If so that's a likely cause.

You could try telneting to port 25 of the smtp server to test it, see here

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No idea, it could be a temporary error at your ISP, it does not really sound like a error with outlook (just yet, without knowing more).

If using POP3/SMTP, try deleting and recreating the account. (warning, DO NOT try this if it is an IMAP account as you may loose saved messages if not careful).

Recreate the account and see if it helps, otherwise, you may just want to wait a while as if nothing has changed, it may not be a problem your end.

Also, double check that you haven't installed (or uninstalled) any new software or Firewalls / Spam filters as these can act as proxys and generally mess up normal computer activities such as sending and connecting to email servers.

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If your Mac and your PC are on the same network (e.g. connected to your home router), the problem must be with the PC. Others pointed out towards changes in firewall settings - this happens sometimes, especially if your computer is in a domain where sysadmins may set some funny policies.

One step I would take is to try to connect to the port 25 at your SMTP server using telnet 25. Note that telnet is by default not available under Vista (use pkgmgr /iu:TelnetClient command that will then ask you to reboot). If telnet displays nothing and waits or if it says 220 host name blah blah service ready, type in HELO and hit Enter (the HELO will be displayed on the screen). The SMTP server will then send answer with 250 host name blah blah. In this case there is some problem with Outlook. It telnet says that it can't connect to the remote host, there is a connectivity problem somewhere. If Mac works in the same network, this is most likely the local firewall.

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-find your smtp server address from account details in your outlook. try to ping or telnet that address from command prompt (see piggymouse's response). -check your firewall or anti virus software, -disable any add-ons in your outlook.

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