Hello Everyone, it is the same for me. It was working before, and stopped right after upgrade. I tried to delete accout and add it once more - without any change. In the same time Thunderbird works just fine. It is also OK when I switch to non-SSL IMAP in Mail.app, so apperantly error is related to SSL. Connection doctor listing below. Apple, please issue fix ASAP!
INITIATING CONNECTION Oct 26 18:28:17.376 [kCFStreamSocketSecurityLevelNone] -- host:balrog.nazwa.pl -- port:993 -- socket:0x6180002d4120 -- thread:0x60800027d500
CONNECTED Oct 26 18:28:17.567 [kCFStreamSocketSecurityLevelTLSv1_0] -- host:balrog.nazwa.pl -- port:993 -- socket:0x6180002d4120 -- thread:0x60800027d500
READ Oct 26 18:28:17.606 [kCFStreamSocketSecurityLevelTLSv1_0] -- host:balrog.nazwa.pl -- port:993 -- socket:0x6180002d4120 -- thread:0x60000007ee80
* OK IMAP4 ready
WROTE Oct 26 18:28:17.607 [kCFStreamSocketSecurityLevelTLSv1_0] -- host:balrog.nazwa.pl -- port:993 -- socket:0x6180002d4120 -- thread:0x6000000737c0
1.13 CAPABILITY
READ Oct 26 18:28:17.645 [kCFStreamSocketSecurityLevelTLSv1_0] -- host:balrog.nazwa.pl -- port:993 -- socket:0x6180002d4120 -- thread:0x6000000737c0
* CAPABILITY IMAP4 IMAP4rev1 UIDPLUS AUTH=PLAIN QUOTA AUTH=PLAIN
1.13 OK completed
WROTE Oct 26 18:28:17.645 [kCFStreamSocketSecurityLevelTLSv1_0] -- host:balrog.nazwa.pl -- port:993 -- socket:0x6180002d4120 -- thread:0x6000000737c0
2.13 AUTHENTICATE PLAIN
READ Oct 26 18:28:17.742 [kCFStreamSocketSecurityLevelTLSv1_0] -- host:balrog.nazwa.pl -- port:993 -- socket:0x6180002d4120 -- thread:0x6000000737c0
+
WROTE Oct 26 18:28:17.744 [kCFStreamSocketSecurityLevelTLSv1_0] -- host:balrog.nazwa.pl -- port:993 -- socket:0x6180002d4120 -- thread:0x6000000737c0
****************************************************************
READ Oct 26 18:28:17.857 [kCFStreamSocketSecurityLevelTLSv1_0] -- host:balrog.nazwa.pl -- port:993 -- socket:0x6180002d4120 -- thread:0x6000000737c0
.13bLOGIN BAD Error in IMAP command received by server.