dsync having problems with @-sign in mailbox names

[Philip Iezzi via dovecot]

Hi there

We are running Cyrus-to-Dovecot migrations using dsync on destination Dovecot 
server (Debian Stretch / latest Dovecot 2.3.7.2 from community repo) like this:

  $ doveadm -o mail_fsync=never backup -R -u <username> imapc:

For imapc configuration to connect to remote Cyrus server, see below [1].
While this works great for hundreds of mailaccounts, dsync fails with the 
following error on accounts with @-sign in mailbox names:

  dsync(<username>): Error: Failed to access mailbox 
INBOX/entrance/admin@example: Invalid mailbox name

I can confirm, Dovecot supports @-sign in mailbox names and no client (tested 
with Thunderbird, Apple Mail, Roundcube webmail) has issue with this character.
Also Cyrus was always supporting this. So I guess this is an issue with dsync. 
Can we somehow tune charset (UTF-8 ?) in imapc configuration?

Thanks,
Philip


[1] /etc/dovecot/conf.d/90-migration.conf:

## IMAP
imapc_host = imap.example.com
imapc_port = 993
imapc_ssl = imaps
imapc_ssl_verify = yes
imapc_user = %u
imapc_master_user = cyrus
imapc_password = **********

imapc_features = rfc822.size fetch-headers
mail_prefetch_count = 20
imapc_list_prefix = INBOX


## POP3
pop3c_host = pop3.example.com
pop3c_port = 995
pop3c_ssl = pop3s
pop3c_ssl_verify = yes
pop3c_user = %u
pop3c_master_user = cyrus
pop3c_password = **********

namespace {
  prefix = POP3-MIGRATION-NS/
  location = pop3c:
  list = no
  hidden = yes
}
protocol doveadm {
  mail_plugins = $mail_plugins pop3_migration
}
plugin {
  pop3_migration_mailbox = POP3-MIGRATION-NS/INBOX
}