A long time ago when I had a .Mac account, I decided to create an alias for a community web site.  I do not remember when but all of a sudden I viewed pop up small windows showing that there was a problem with one of my aliases.

 

There may be TWO issues to contend with to fix this problem.  I will only address the SMTP Server issue in this post.

First issue:
Here is the major clue to finding the cause to this problem.
The SMTP server “smtp.me.com” rejected the password for user “xxxxxxxxxx”

So we are looking for SMTP server items to examine and resolve.  I really do think this was the solution.  I found that I had to delete a number of SMPT Servers.

SMTP Server Procedure

Open Mail and select that application so the computer screen shows that menu at the top of the screen.

Next to the apple icon in the upper left corner of your computer screen is the menu option Mail and when your mouse clicks on that word which will reveal a pull down menu selection.  Select Accounts from the list at the top of the splash screen.  Make sure that Account Information tab is selected.  Look down to the line showing this label: “Outgoing Mail Server (SMTP)”.  Look at the far right of the information within that selection.  You should see up very microscopic up and down arrows.  This means that mouse clicking this line of text should display more offerings.  My computer showed 31 SMTP servers.
At the bottom of this long list (long for me) is the option to “Edit SMTP Server List …”

Choose “Edit SMTP Server List …”.  I found that I had 2 .Mac SMTP servers and 2 .Me SMTP servers on the list.  Question is, which one is causing the problem.  I never knew for sure.  I will delete one after another until I get results.  Select the server you want to get rid of and then mouse click on the negative “-” sign at the bottom of the server listing.  (I modified the server list below to hide my servers).

 

Whenever I tried to click on the minus sign to delete a SMTP server – I got an error message.

 

 

I selected .Mac Account but I got an error message.
I selected smtp.me.com and got an error message.
I selected another smtp.me.com and it completed.

So, in other words, I selected one after another SMTP server and wanted to add each one in turn to be deleted.  When I got to 3 servers it worked.  Problem for you is correctly selecting the correct ones.

I was asked if I wanted to save and I agreed.  Then all of a sudden a whole bunch of  e-mail messages popped up with “Cannot send message” splash screens.

 

I chose “Always Use This Server” after selecting iCloud as the correct new server.

I thought everything was solved because I no longer had the pop up small screen showing that there was a SMTP Server error.   But, all of a sudden, I had a whole bunch of very old e-mail messages that showed with the list of SMTP servers to choose from to sent the message.  I just deleted each message because they were so old and most embarrassing to have them sent now.

I rebooted Mail and I never viewed a pesky window telling me that the “SMTP server “ rejected the password.

I would like to thank Awesome Dude, the technical support fellow at Apple Computer who stayed with me for over an hour trying to figure this out.