If you get the above error, make sure that you have CONFIG_NFSD_V4 enabled in your kernel config.

Running glxinfo gave the error

libGL error: failed to open drm device: Operation not permitted

and strace showed:

open("/dev/dri/card0", O_RDWR|O_LARGEFILE|O_CLOEXEC) = -1 EPERM (Operation not permitted)

The fix is to add

--property='DeviceAllow=/dev/dri/card0 rw'

to the systemd-nspawn command line

# iftop -i eth0
interface: eth0
pcap_open_live(eth0): eth0: socket: Address family not supported by protocol
# dmesg | tail
[595841.213279] iftop uses obsolete (PF_INET,SOCK_PACKET)

Make sure that CONFIG_PACKET and CONFIG_PACKET_DIAG are enabled in your kernel config

$ diff .config.old .config
3c3
< # Linux/x86 4.4.70 Kernel Configuration
---
> # Linux/x86 4.4.89 Kernel Configuration
635c635,636
< # CONFIG_PACKET is not set
---
> CONFIG_PACKET=m
> CONFIG_PACKET_DIAG=m
$ 

If you are trying to unmount a device, you get the error "device is busy", but lsof/fuser don't show any process holding the device open, it could be that a file on the device is associated with a loop device.

try

# losetup -a
/dev/loop/0: [0821]:25280514 (image) offset=55273881600
# losetup  -d /dev/loop0 
# umount /mnt/tmp

(updated, 2016-11-01)

dmsetup info | grep loop
dmsetup remove loop0p2
dmsetup remove loop0p3

may help too

(previously)

# ls -l /dev/mapper/
# dmsetup ls
# dmsetup info loop0p2
# dmsetup remove loop0p2
# losetup -a
# losetup -d loop0
# umount /mnt/tmp

(updated for Malwarebytes, 2016-02-04) When working with friend's PCs, here are some applications I install:

• Secunia PSI Personal Software Inspector
(updated 2018-03-14 - this will soon be discontinued)
• (Added 2013-03-04) Filehippo Update Checker
• (Added 2013-11-22) Colleagues also recommend Ninite
  reviewed at The Register
• Grisoft AVG free anti-virus
• Lavasoft Ad-Aware Not any more - see this article (2011-11-13)
• Spybot-S&D
• (added 2008-02-10) And one more link: Windows Sysinternals
• (Added 2011-12-27) SANS ISC recommend MalwareBytes free anti-virus, OTOH: security issues,
• (Added 2013-03-04) The UK government's advice

If you do not have a static IP address, use your ISP's mail relay (smarthost).

Make sure that there is a reverse DNS (PTR) record for your IP address. Make sure that it is not generic (e.g. wylie.me.uk, rather than client-12-34-56-78.adsl.example.org). You will need to contact your ISP to do this. (See RFC1912 section 2.1)

Make sure that doing a PTR lookup, then doing an A lookup on that domain name gives the original IP address, e.g. $ host 82.68.155.89 89.155.68.82.in-addr.arpa domain name pointer wylie.me.uk. $ host wylie.me.uk wylie.me.uk has address 82.68.155.89

Get your ISP to SWIP your IP allocation, so that a whois lookup shows that it is allocated to you.

Make sure that your ISP has not listed your IP address as dynamic with any DNSBL, e.g. dul.dnsbl.sorbs.net. If necessary, get your ISP to contact the DNSBL maintainers and ask for your IP address to be delisted.

Make sure that your HELO/EHLO string is a FQDN (Fully Qualified Domain Name), e.g. "wylie.me.uk", rather than "NTSERVER", as required by RFC 2821 section 4.1.1.1

Further reading (This was originally written in 2006, some of these links may be out of date, I'm afraid):
SORBS
Spamhaus reverse DNS
Spamhaus Dynamic IP lists
Yahoo mail help
AOL Technical Standards for E-mail Delivery
AOL Reverse DNS
Messaging Anti-Abuse Working Group Sender Best Communications Practices