With the updation of Mac OS from Yosemite to EL Capitan , Users need to transfer Mac mails to new Mac OS. There is an issue of mails getting crashed during transfering emails in Mac mail from older Mac OS.
Here are some probable reasons with possible solutions for crashing of mails :
There may be possibility that you have your old emails in broken format in one of the files. For that: Follow Library >> Mail >> V2 >> Mail_account_name. Then, check each folder of the mail and move some of the mails into another folder and select half of the folders at single time until you are able to find the group that it crashes on. Repeat the process until you find a bad mail.
There might be some emails with MacRoman encoding with attachments containing non-ascii characters in their file name. To avoid this issue, You need to rename the attachments in .emlx file of Apple mail.
If, you already have these mails in your email archive. In this case, you have to check your email archive.
Fortunately, relevant solutions are provided for above mentioned issues. However, these solutions are very tedious and time taking as well.
So, there is also an alternate and less time consuming method:
You might had a POP based account or you had local mail folders, Opt for an IMAP service and open an account.
Create a folder and transfer your local mail folders in that.
Then, Configure your new email client from new machine to that particular IMAP account.
After synchronization of messages you can drag them back to the local folder.
In case of POP account, you can choose the option "Leave Messages to Server" while connecting to the email client.
After that you can efficiently connect your account to any email client.