Mail preference window is very, very wide and can't adjust

Here's what worked for me: Delete all the signatures; then open Mail with the Shift key. The preference pane cannot be resized by default. Engineering has the ball - I spent three hours on the phone with tech support.

Had the same issue. I had 3 custom signatures for 3 accounts. I deleted all three and re-entered them. Cured the Preference Pane issue.

I had the same issue with the compose mail pane a while back and found the same solution. Apparently when entering signature info there was something in it, a return, page break (I composed in word and pasted them the first time) that caused the issue then. Decided to try it and it worked...

I had this issue: Mail Preferences pane too wide past sides of screen. No green dot. No adjustment symbol at window edge or corner.

This started yesterday after I logged into iCloud and re-enabled an iCloud alias that received spam. Today I contacted Apple about issue and they were not familiar. I thought about what I had done differently lately and decided to log back into iCloud. Selected Mail. Right clicked a spam message that came in via the problematic iCloud alias. Saw that there was a bunch of code in frameworks showing things like "don't allow user to delete or adjust etc etc...". That did not look like anything friendly from Apple... I immediately applied rule to remove the spam, and DISABLED the problematic iCloud Alias. Closed browser. PREFERENCE PANE restored to normal functionality.

Great, but I don't have an iCloud alias and I almost never send mail from iCloud and never received any there. I checked all the mail in iCloud and there was nothing. I searched for "frameworks" in my mail and only three messages came up: yours and two from 2015. This problem started after the upgrade to High Sierra 10.12.

Do you receive any spam at all? The type with the links to this or that? Addresses you do not recognize? Some from European domains? Even if you do not have iCloud, see if removing an email address that is spammed [phishing etc] from Mail accounts will fix it.

I right clicked on the spam message and chose to display sources... that is when I saw the code

Sure, I get lots of Spam. It gets sent to Junk and then deleted. I guess I don't understand what you're suggesting. I have 4 email accounts including iCloud. Are you suggesting that an address from a spammer that may have been saved can cause this? How is that possible?

Highlight the "3 or 5 line summary" Go to menu and click View then scroll down to Message and select View Source. Take a good look at the code in the spam message... If you received the same as me, then it will have lines in it about not letting the user change preferences or something to that effect". My theory is that someone and it appears to be from Europe, is writing code to infiltrate Mac Mail... JUST TRY THIS - - - Get rid of the email addresses that are inundated with Spam. Clean that up. Reboot in safe mode a few times after checking the Mail Preferences over and over until you see the small Preferences pane return. Then you are done. Get rid of Spam infiltrated emails ! I had only the one Spam infiltrated email - I re-activated it - the problem with the Preference pane started - I de-activated it [removing the incoming spam infiltrators first] and - re booted in safe mode a couple times - that worked immediately for me - Problem is still gone

I do not know of a way to search the source for all the emails completely/overall but Apple might know... I hear you. If you need to remove the 10s of 1000s emails but save them and try to work on them later, you can call Apple and tell them about this problem people are having [I reported it directly to them yesterday and a little while ago] and they will assist you to archive the emails to a large thumb drive or ext hard drive. Then deactivate the bad email addresses and do not use them anymore in your mac mail. This can be done in less than an hour. I would get rid of any and all potential spam that is in your system. Please call Apple tech and ask them if this is a good idea. I think they will agree.

Well...I have had this irritating problem also, 'Inbox' window way too wide.

However...I have found a cure so simple I can't believe it will last! In your 'Inbox' where you have 'From' 'Subject' 'Date Received', put your mouse just to the left of 'Subject' and 'Date Received' and drag the small bar to the left and that's it! So far my windows have stayed the same size.

This worked for me too - many thanks!

Exact same issue for me. I can resize the columns but the Prefences window still remains incredibly wide with no way to reduce the overall width.

I have this problem. I can't even resize the window; the cursor won't grab a corner or a side.

I have 10's of thousands of saved emails. I won't live long enough to search all of them. Do you know of a way to search the source?

Preference pane settings are probably in the code for the mail app. I'll see if the tech boys can help me. Thanks.