Checking or deleting email on large imap accounts is a show stopper. Not only does it take forever but you cant even work with other accounts as sylpheed is locked up untile the original imap session is over. It took 10 mins. for me to delete 1500 emails via.. imap. I tried the same with other mail clients with only took sec. and was able to admin other accounts with the client. also i have noticed that after deleting all the contents of "inbox" sometimes sylpheed shows (2) messages remaining despit the dir being empty.
Fixed in 1.9.11cvs64