Hi, I have been having problems with mail hang since upgrading to the 10.10.3 as well.
Below is the log as instructed by you above:
19/4/15 4:14:21.579 pm com.apple.xpc.launchd[1]: (com.apple.imfoundation.IMRemoteURLConnectionAgent) The _DirtyJetsamMemoryLimit key is not available on this platform.
19/4/15 4:14:22.341 pm discoveryd[77]: Basic DNSResolver UDNS Send(): UDP Sendto() failed to DNSNameServer 8.8.8.8 Port 53 errno 51, fd 60, ErrLogCount 5 ResolverIntf:4
19/4/15 4:14:23.439 pm discoveryd[77]: Basic DNSResolver UDNS Send(): UDP Sendto() failed to DNSNameServer 8.8.8.8 Port 53 errno 51, fd 64, ErrLogCount 6 ResolverIntf:4
19/4/15 4:14:24.542 pm discoveryd[77]: Basic DNSResolver UDNS Send(): UDP Sendto() failed to DNSNameServer 8.8.8.4 Port 53 errno 51, fd 69, ErrLogCount 7 ResolverIntf:4
19/4/15 4:14:25.618 pm discoveryd[77]: Basic DNSResolver UDNS Send(): UDP Sendto() failed to DNSNameServer 8.8.8.4 Port 53 errno 51, fd 60, ErrLogCount 8 ResolverIntf:4
19/4/15 4:14:25.672 pm Mail[771]: {"status":"400","schemes":"Bearer","scope":"https://mail.google.com/"}
19/4/15 4:14:25.739 pm Mail[771]: {"status":"400","schemes":"Bearer","scope":"https://mail.google.com/"}
19/4/15 4:14:26.251 pm Mail[771]: {"status":"400","schemes":"Bearer","scope":"https://mail.google.com/"}
19/4/15 4:14:26.425 pm Mail[771]: {"status":"400","schemes":"Bearer","scope":"https://mail.google.com/"}
19/4/15 4:14:26.581 pm Mail[771]:
...... more ...
19/4/15 4:14:35.382 pm Mail[771]: {"status":"400","schemes":"Bearer","scope":"https://mail.google.com/"}
19/4/15 4:14:36.095 pm Mail[771]: {"status":"400","schemes":"Bearer","scope":"https://mail.google.com/"}
19/4/15 4:14:36.128 pm Mail[771]: XOAUTH2 requires user
19/4/15 4:14:36.128 pm Mail[771]: Failed to start the SASL connection
SASL(-1): generic failure: XOAUTH2 requires user
19/4/15 4:14:36.295 pm Mail[771]: {"status":"400","schemes":"Bearer","scope":"https://mail.google.com/"}
19/4/15 4:14:36.308 pm Mail[771]: XOAUTH2 requires user
19/4/15 4:14:36.308 pm Mail[771]: Failed to start the SASL connection
SASL(-1): generic failure: XOAUTH2 requires user
19/4/15 4:14:36.787 pm Mail[771]: XOAUTH2 requires user
19/4/15 4:14:36.787 pm Mail[771]: Failed to start the SASL connection
SASL(-1): generic failure: XOAUTH2 requires user
19/4/15 4:14:36.811 pm Mail[771]: XOAUTH2 requires user
19/4/15 4:14:36.811 pm Mail[771]: Failed to start the SASL connection
SASL(-1): generic failure: XOAUTH2 requires user
19/4/15 4:14:36.836 pm Mail[771]: XOAUTH2 requires user
19/4/15 4:14:36.839 pm Mail[771]: Failed to start the SASL connection
SASL(-1): generic failure: XOAUTH2 requires user
19/4/15 4:14:36.865 pm Mail[771]: {"status":"400","schemes":"Bearer","scope":"https://mail.google.com/"}
19/4/15 4:14:36.903 pm Mail[771]: XOAUTH2 requires user
19/4/15 4:14:36.903 pm Mail[771]: Failed to start the SASL connection
SASL(-1): generic failure: XOAUTH2 requires user
19/4/15 4:14:37.481 pm Mail[771]: {"status":"400","schemes":"Bearer","scope":"https://mail.google.com/"}
Appreciate it if you can provide some suggestion as to how to fix the problem.
Thanks.