When you post a URL on your site, I'm 99.9% sure you have it set to be converted to the page title. I'm trying to confirm whether you've installed an add-on to do this or if you've enabled the "Convert URLs to Page Titles" option (in 1.5). If it's the former, you may need to contact the author; if it's the latter, then it's likely to require a tweak to how to we handle this in a future release.
I would completely disable the add-on to see if that helps. It's relevant that the message you're sending has a link to a file with a size that roughly corresponds to the amount of memory being allocated in that error. The default option will probably still be affected to some degree, but it shouldn't be affected as much. For now, I'd recommend disabling the add-on but not enabling the default version until after the messages are sent to avoid causing problems.
I would note that it's very likely that some of your messages have been sent so there's potential for duplicates (some may have already been duplicated if the page has been refreshed). It's tricky to know who they have been sent to exactly because you're deleting the conversation from your inbox. Determining this may require access to the DB, though as it stands, there isn't an obvious way to skip those users on the next run.
If you want me to try to determine this, please submit a ticket via your customer area. I would need phpMyAdmin/query access.
While I can't fix the add-on that triggered this, I have adjusted the code that caused this indirectly. Autolinking is now down immediately after the conversation is entered, so the preview will reflect this as well.