please try if following helps:
create /etc/systemd/system/dovecot.service with content:
.include /usr/lib/systemd/system/dovecot.service
[Unit]
After=autofs.service

and reboot. Does it help?

(In reply to comment #1)
> Does it help?
No, it doesn't.

# cat /etc/systemd/system/dovecot.service
.include /usr/lib/systemd/system/dovecot.service
[Unit]
After=autofs.service

After rebooting, user "corsepiu" logging-in into xfce via gdm and launching thunderbird:

# systemctl status dovecot.service
dovecot.service - Dovecot IMAP/POP3 email server
	  Loaded: loaded (/etc/systemd/system/dovecot.service; enabled)
	  Active: active (running) since Fri, 27 Jul 2012 16:32:19 +0200; 5min ago
	Main PID: 834 (dovecot)
	  CGroup: name=systemd:/system/dovecot.service
		  ├ 834 /usr/sbin/dovecot -F
		  ├ 868 dovecot/anvil
		  └ 869 dovecot/log

Jul 27 16:32:19 beck dovecot[834]: master: Dovecot v2.1.8 starting up (core...d)
Jul 27 16:32:19 beck dovecot[834]: master: Warning: /run/user/corsepiu/gvfs...nt
Jul 27 16:32:45 beck dovecot[869]: imap-login: Login: user=<corsepiu>, meth...o>
Jul 27 16:32:45 beck dovecot[869]: imap(corsepiu): Error: chdir(/users/cors...ks
Jul 27 16:32:45 beck dovecot[869]: imap(corsepiu): Error: user corsepiu: In...iu
Jul 27 16:32:45 beck dovecot[869]: imap(corsepiu): Error: Invalid user sett...n.

OK, I tried to reproduce this. 

Environment:
mkdir /home/auto
adduser -d /home/auto/afsuser afsuser
passwd afsuser
new /etc/auto.home with following line:
afsuser -fstype=ext3,loop,rw :/home/disk.img
added "/home/auto /etc/auto.home" to /etc/auto.master
chkconfig dovecot on
chkconfig autofs on
dd if=/dev/zero of=/home/disk.img bs=1M count=100
mkfs.ext3 -F /home/disk.img
mount /home/disk.img /mnt -oloop
mv /home/auto/afsuser /mnt
umount /mnt

default dovecot configuration, set mail_location=maildir:~/Maildir

configure thunderbird to use afsuser@localhost:143

1) Reproduce - force autofs started *after* (note "Before=") dovecot:
# cat /etc/systemd/system/dovecot.service 
.include /usr/lib/systemd/system/dovecot.service
[Unit]
Before=autofs.service

reboot, add enforcing=0 to kernel cmd line

start thunderbird -> "Login to server localhost failed."
service dovecot status:
master: Dovecot v2.1.9 starting up (core dumps disabled)
master: Warning: /misc is no longer mounted. See http://wiki2.dovecot.org/Mountpoints
master: Warning: /net is no longer mounted. See http://wiki2.dovecot.org/Mountpoints
master: Warning: /home/auto is no longer mounted. See http://wiki2.dovecot.org/Mountpoints
imap-login: Login: user=<afsuser>, method=PLAIN, rip=::1, lip=::1, mpid=884, secured, session=<u5QeJdzHfAAAAAAAAAAAAAAAAAAAAAAB>
imap(afsuser): Error: user afsuser: Initialization failed: Namespace '': Can't create mailbox root dir /home/auto/afsuser/...ount remove
imap(afsuser): Error: Invalid user settings. Refer to server log for more information.

-> reproducible

2) Try the fix - start dovecot after autofs:
# cat /etc/systemd/system/dovecot.service 
.include /usr/lib/systemd/system/dovecot.service
[Unit]
After=autofs.service

reboot

cat /var/log/boot.log:
         Starting Automounts filesystems on demand...
.........
[  OK  ] Started Automounts filesystems on demand.
         Starting Dovecot IMAP/POP3 email server...
[  OK  ] Started Dovecot IMAP/POP3 email server.

so dovecot really is started after autofs

thunderbird -> Login failed again

service dovecot status:
dovecot: master: Dovecot v2.1.9 starting up (core dumps disabled)
dovecot: imap-login: Login: user=<afsuser>, method=PLAIN, ...
dovecot: imap(afsuser): Error: chdir(/home/auto/afsuser) failed: Too many levels of symbolic links

WTF?

# cat chdircheck.c 
#include <unistd.h>
#include <stdio.h>
#include <errno.h>

int main(int argc, char **argv)
{
  int r=chdir("/home/auto/afsuser");
  fprintf(stderr,"Return code is: %d, errno=%d : %m\n",r,errno);
  return r;
}

gcc chdircheck.c -o /home/chdircheck
to /etc/.../dovecot.service added:
[Service]
ExecStartPre=/home/chdircheck

reboot
service dovecot status: failed
/var/log/messages: 
chdircheck[608]: Return code is: -1, errno=40 : Too many levels of symbolic links

seems there is some problem with autofs. I'll investigate and file a new bug.

> seems there is some problem with autofs. I'll investigate and file a new bug.

the bug seems to be in systemd or kernel : bug #851131

That systemd bug won't be fixed in Fedora 17 nor 16, but there should be workaround for that bug.

please try if following helps:
create /etc/systemd/system/dovecot.service with content:



.include /usr/lib/systemd/system/dovecot.service
[Unit]
After=autofs.service

[Service]
PrivateTmp=false




and reboot. Does it help?

(In reply to comment #5)
> That systemd bug won't be fixed in Fedora 17 nor 16,
Is anybody surprized, why Fedora is loosing users? I regret, but such kind of regressions, definitely contribute their share to the "great Fedora experience".

> but there should be
> workaround for that bug.
... 
> please try if following helps:
> create /etc/systemd/system/dovecot.service with content:
...
> and reboot. Does it help?

Seems so. At least, dovecot appeared working properly after 2 reboots for me.

Thanks for you efforts.

Should be fixed in dovecot-2.1.10-2.fc17 I did not use 'After=autofs.service', because for my testing it was not necessary. Only the systemd workaround was required.

dovecot-2.1.10-2.fc17 has been submitted as an update for Fedora 17.
https://admin.fedoraproject.org/updates/dovecot-2.1.10-2.fc17

Package dovecot-2.1.10-2.fc17:
* should fix your issue,
* was pushed to the Fedora 17 testing repository,
* should be available at your local mirror within two days.
Update it with:
# su -c 'yum update --enablerepo=updates-testing dovecot-2.1.10-2.fc17'
as soon as you are able to.
Please go to the following url:
https://admin.fedoraproject.org/updates/FEDORA-2012-17584/dovecot-2.1.10-2.fc17
then log in and leave karma (feedback).

dovecot-2.1.10-2.fc17 has been pushed to the Fedora 17 stable repository.  If problems still persist, please make note of it in this bug report.