Reply works, but address book entry doesn't

Question

I have an odd problem that's making me a litttle batty. I have an friend I e-mail regularly and--since recently switching to Mail from Eudora--I've been finding that some of my messages are not getting through.
I'm not 100% certain, but it seems that when I send a new message that's addressed from the Address Book entry, the message is never received. But when I REPLY to a message from this same recipient--same address--the message goes through just fine.
I've reviewed the address again and again and it's entered correctly.
When I look at the Long Header, one difference I note between the received and no-received maessages is under Content-Type.
A successful message has:
Content-Type: multipart/alternative; boundary=Apple-Mail-8-603300258
And a failed message:
Content-Type: text/plain; charset=US-ASCII; format=flowed
Another difference is that the failed messages--the ones that originated with me have something called:
Content-Transfer-Encoding: 7bit
While the messages I reply to don't have that line at all.
The recipient--by the way--is at work at a large organization. So I wonder if their mail servers are blocking messages with certain kinds of encoding?
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OK, OK...this just in. While I was composing this message, I was also franticly corresponding with the adressee in question, trying to troubleshoot this. I may have solved my own problem, but I'm going to post this anyway... In hopes of solving someone else's problem, and possibly getting some responses that will enlighten me further (cuz I just don't get this).
On a wild guess/hunch, I went into PREFERENCES/COMPOSING and changed the Message Format: from Rich Text to Plain Text. The next test message got through! One difference (if it matters??) I found in the Long Header was the
Content-Type changed from:
text/plain; charset=US-ASCII; format=flowed
to
text/plain; charset=US-ASCII; delsp=yes; format=flowed
So anyway...I'm completely flummoxed. Any ideas, clues, answers out there? I'm not going to trust yet that this problem is completely solved. So I'll leave this as an open question for now.
verbosely yours,
Roger
Mac G5   Mac OS X (10.4.5)  

Answer 1

Hmmm... there's nothing fancy about the signature (no bolds, itals or other styles or colors). But does that mean that it IS plain text? My setting when I made the signature(s) was Rich Text, I'm sure. So would that apply, even though it's plain for all intents and purposes? And if it's not. How is it changed to Plain Text?
As far as the SMTP settings. I tried one change last night. I changed the Server settings from "smtp.ispname.net:username" to "smtp.ispname.net:[email protected]". When I tested a message to the address/server I've had the most problems with, the message arrived successfully. But I don't think that proves anything as yet. The nature of the problem has been mysteriously intermitent anyway. (that message had my regular signature as well).
AS I'd mentioned, I've been moving stuff out of my Sent Mail folders to keep things organized and for reference (Is doing that OK? is Sent Mail suppposed to be left wher it is for some reason?). But on informal investigation, I would say that all of the messages I've sent--original or reply--HAVE shown up in the Sent Mail folder.
The discussion thread I'd been involved in with you before is:
http://discussions.apple.com/thread.jspa?threadID=347863
However, I haven't added any posts since way back then.
Thanks you for any insights you can give me with this. If you're not on Apple's payroll, you should be.
Roger