Suddenly cannot send email through any SMTP server

Ok, so I've read all the posts and not much of it has made sense to me...

I am unable to send email from Mail via my MobileMe account, which is SO frustrating. I can send mail successfully from the web, just not from my Mac.

Incoming mail is fine but for outgoing mail, I get the following error message: "5.7.0 From address is not one of your addresses."

It then asks me to choose another mail server from the drop-down box, which there is only one choice - which is smtp.mac.com - and it still doesn't work. I am a basic user so this all is very confusing to me. Also I notice that in my settings, it's a Mac account rather than a MobileMe account yet I can't change this?? Does it make a difference?

Can anyone please help??? I'm so lost!!

Your account should be saying stmp.me.com, not mac.
Go to this forum topic and see if it can help you.

http://discussions.apple.com/thread.jspa?messageID=7839469

Good luck.

THIS Solution worked for me:

"Under the Apple, open your System Preferences, select the Network icon.

Next, highlight you Ethernet, and click the Configure Button at the bottom of that screen.

The next screen has a row of 5 buttons near the top. The last one is ETHERNET.

Apparently, By Default, the next screen has the Ethernet set to Configure: Automatically.

Dan figured out that his wireless router would not accept Packets larger than 1492.

When I clicked on the Automatic Button it revealed a drop down option to Confiure: Manually.

This is your Final window to deal with.

The radio button for the Maximum Packet Size was highlighted STANDARD (1500)

Remember DAN wrote that his Router would NOT accept greater than 1492.

I selected the Custom: |EDITABLE FIELD| (Range 72 to 1500), radio button,

Wouldn't you know that by changing my editable field to 1492

MY EMAILS WILL SEND"

BUMP

I a complete novice when it comes to working with Mail - I used it every day, probably very ineffficiently, but it works for me. I've been reading posts on this topic for hours and am no closer to even understanding what is being written. I keep reading about these ports 25, 465 and 587; the latter is most often mentioned. What are these ports? Is there somehwere I can read about them? And what does it mean that the ISP is "blocking" the port, and why would he do this? I sent my daughter in Belgium a 15" aluminum PBook recently. She had no problem setting up her internet connection, and everything works, except she cannot send mail, and it obviously has something to do with ports. The moment she clicks "Send" she gets a msg that the connection on the default port timed out. What is this default port, and why would it be timing out. I guess I need to specify a different port, but how do I do this? Thanks anyone.

Because my daughter's computer I just shipped to her in Belgium cannot send mail, I spent countless frustrating hours trolling the posts for a solution. I didn't find one although many were offered for specific situations, and these were more or less unintelligible to me because of my limited knowledge of ports and the like. I did a post a little earlier about her very specific port-related problem. Then I decided to try something really duh. I went to Mac Help in the Finder, and typed in "What is a port", which yielded a whole slew of things about ports but not the type I was interested in. However, at the bottom of the help screen there were a bunch of articles, one of which was entitled "Mac OS X 10.5: Can receive email, but not send email" (support article #TS1307), and which discusses 2 issues. One deals with getting a msg from Mail "Cannot send message using the server example SMTP", and the other treats a dimmed send button. Note that this relates only to Leopard but may solve the problem for at least some of you. Moreover, it does show that Apple is aware of this problem under Leopard, since one of the solutions is downloading 10.5.5 (which, by the way, may still not solve the problem). Anyway, I'd thought I'd pass this along to the community. In the mean time, I've told my daughter to get a Yahoo mail account, which according to the posts, may still not solve it. We'll see.