16.04 with dovecot/postfix/sieve - what should permissions be?

Hi

Brand new, clean install of 16.04, specifying LAMP and Mailservers at the tasksel installation screen.

Everything is working well EXCEPT for mail delivery:

Jun 28 11:31:37 mallard postfix/pipe[11765]: 634B463E78: to=<[email protected]>, orig_to=<[email protected]>, relay=spamassassin, delay=1.2, delays=0.87/0/0/0.34, dsn=2.0.0, status=sent (delivered via spamassassin service)
Jun 28 11:31:37 mallard postfix/qmgr[11714]: 634B463E78: removed
Jun 28 11:31:37 mallard postfix/qmgr[11714]: 881F265707: from=<[email protected]>, size=44213, nrcpt=1 (queue active)
Jun 28 11:31:37 mallard dovecot: lda(janice): Error: setegid(privileged) failed: Operation not permitted
Jun 28 11:31:37 mallard dovecot: lda(janice): Error: sieve: msgid=<SN2PR0801MB223884DA7B189A79DA3A4CE2B5220@SN 2PR0801MB2238.namprd08.prod.outlook.com>: failed to store into mailbox 'INBOX': BUG: Unknown inteal error
Jun 28 11:31:37 mallard dovecot: lda(janice): Error: sieve: Execution of script /sieve/before/spam.sieve was aborted due to temporary failure
Jun 28 11:31:37 mallard postfix/local[11769]: 881F265707: to=<[email protected]>, relay=local, delay=0.02, delays=0.01/0/0/0.01, dsn=4.3.0, status=deferred (temporary failure)

The only way I can get around this is to chmod o+w /var/mail, which doesn't sound ideal.

So, can anyone suggest: the correct permissions for /var/mail, and anu user or group settings i should have in the dovecot config.

Many thanks

Jim