I find that anytime a PM I am authoring gets large or has a significant number of quotes in it (as in the case of a PM conversation going back and forth), a reply that I send seemingly goes out but then is never delivered.  It doesn't get to the other party and it doesn't show up in my Sent items, even though I selected that option.  I get the Message Sent confirmation screen, but no dice...and the back button does not get you back to the editor, so you are screwed.  Whatever you typed has been lost.  

I have been selling some items in the EE in recent weeks and this has happened to me at least a dozen times...and it sucks.

Same here, I checked the "save to sent folder" but no IM was saved there, I checked in here and when I tried deleting all the quotes it finally went through and saved to the sent folder.....

This was happening to me for awhile too.

It's happening to me too.  I'm not sure if the recipients are getting them.

@Goatboy

Just confirmed the two recipients did not get my replies.  I have no idea what's going on but they were long messages so that's disappointing.

And no telling how long it's been happening.

I think I have the same issue.  IMs getting sent but not sent.  Had a buyer think I am ghosting him, but its because none of my ims are being recieved.