I have had this exact same issue on a brand new customized iMac with upgraded SSD Drive and other upgrades for which you pay a fortune. I spent more than a month with Mac tech support and was shifted around to five different people, every one of them assuring me that they were going to be on this until it was resolved. Then they don't call you for your "scheduled" call back so you call them and have to start over with a new person. Their support is a joke. I spent hours at a time with screen sharing where they recorded and dowloaded reports and other data that was sent to their "engineers." I have two business emails and a personal email and it was happening on all just as you state. The first email would work fine, but on the subsequent emails the whole mac mail program would lock up and I'd have to force quit....I had to click on another program to get out of that window in order to do so. I also explained to them that I did not have the issue when working in webmail (business or personal) or when working on my Macbook Pro (also brand new) which had the exact same email accounts set up on it.
After reviewing the files that were uploaded to them following my first screen sharing session, I was told the "engineers" diagnosis came back and determined it was my business email server settings. They said the MX records were not correct and were diverting data to other servers.... they said it was being diverted to twocows.com. I have an email consultant I work with who set up my business emails and he checked all of the server settings and they were all correct. On my next call with tech support I explained this and reminded them that this was happening on both my business and personal emails (which go through two different servers) and also reminded them that I had no problem when working in webmail or on my macbook pro (also brand new) which has the same email accounts set up. My point was that if they thought about it, the "engineers" diagnosis did not make sense to begin with. They tried all kinds of other ridiculous things like "rebuilding" my mail...or at least that's what I was told we were doing....basically opening up folders in utilities and taking out certain mail folders. We tested immediately after and the problem persisted and I was told that it can take a while for the changes to filter through and that I should work with it a day or two to see if the problem didn't resolve. Of course it never did.
The difference between my iMac and Macbook was that I paid for data migration on my iMac which included my contacts (address book).
And just like you, that's where the issues occur. When you start typing out an email address, the mail app gives suggestions from your contacts based on similar email addresses, but once the data you're typing no longer matches any existing contact info., it should stop suggesting and take the email address you are typing. In my case, even after I typed the entire unique email address my computer would pop in one of the other similar email addresses from my address book and then it would freeze up. I explained this to the last tech support person I was working with and said I believed it had to be an issue that was occurring between the mail and the address book. At his request, I sent him a detailed email explanation with examples for him to forward to the engineers. I never heard back from him even after sending emails and leaving phone messages on his phone line (it's been 3 weeks now and I'm still waiting)
In the meantime, I finally ended up at my local genius bar and they couldn't figure it out either, but agreed it was an issue between mail and contacts. In the end the only thing we could do was disable my existing contacts so they would not link to my mail program. That did fix the issue, but it's really just a bandaid.
Sorry I don't have a fix for you. I'm replying in the hopes that Apple will see this and maybe try to figure out what the real issue is.
