Balsa has been tried, Thunderbird has been suggested, and Barry is working on Claws for the upcoming release.
Not all linux email clients make it seamless to convert a local email mail directory to a different client.
Seamonkey uses mbox, as does Thunderbird, but Tbird has it's own way of implementing mbox, so not everything converts straight across.
Issues I had with migration of local stored email included retaining the previous directory structure, especially if one creates subdirectories. A few different solutions to migration were tried and many services and scripts are available.
One easy migration method often mentioned, is to download mail to the new client from the server. This is acceptable if most of your mail is still on the server. Uploading your local mail to the server from with your old client and then downloading to the new client is a way around converting archive formats, but can be problematic for large archives or those not wishing to relocate their old emails to the cloud, even if temporarily.
claws uses the MH format (and supports mbox with a plugin)
sylpheed uses MH (claws is a sylpheed fork)
I finally found and used a mozilla extension to import my local mail folder to seamonkey. It may be useful should you wish to convert your local folder to MH.
https://addons.thunderbird.net/en-US/th ... porttools/
and there is a newer version I have not tried,
https://addons.thunderbird.net/en-US/th ... ttools-ng/
At this point in time, I use a portablized seamonkey 2.46 on a data partition only to maintain my local email archive, and use webmail (imap) for day to day use, sweeping mail to seamonkey wiyh pop3 occasionally. I do not expect imap, pop3 or smtp to change as fast as http, so hoping it will work for a while yet.