I have a problem with the new beta 2 mail - beta 1 was the same: I have several accounts - gmail and icloud. These accounts are quite small and work well. There is one gmail-account which is around 30GB and mail is freezing after activating that account. When I deactivate, mail works without problems.
Any ideas how I can bring this account into my mail - yosimitee mail worked without a problem.
30GB is pretty big, but would still have been workable under Yosemite. Have you checked network traffic while Mail is apparently freezing? It may be that non-UI processes are still in operation trying to sync the account and just need time...
I saw a similar issue to this on beta 1 and reported a bug on it. Specifically I was able to setup my large GMAIL account and it pulled mail in but then stuck on Checking For Mail. This continued and if you looked at the activity window it would look like it was running an inventory on the GMAIL message list (number of total messages) and start to download but never completed even when left for over 24 hours. I still have the same issue on my beta 1 machine for El Capitan now but will be trying this in beta 2 shortly.
The more I read, the more I feel increasingly fortunate that El Capitan DPs are behaving themselves so (relatively) well for me 😀.
I can confirm that my big GMAIL account that was having issues with El capitan beta 1 (still is on my other test machine) is working fine under the clean install of El Capitan beta 2.
Thats interesting - I am able to open mail without that one account, when I activate it again mail freezes ...
Any idea what I can do?