[NF] Linux SMTP/POP Servers? - ProFox

16 Jun 2016


      We've been running an internal email system that is built into the 
ClearOS operating system (a version of CentOS that functions as a 
SAMBA 3 domain controller) for many years. Along with the ClearOS 
SMTP/POP server, we've been running MailMan on the same machine to 
provide an internal listserv capability. Unfortunately, I'm going to 
have to discard ClearOS and go with a Windows AD domain server. I 
want to preserve the internal email and listserve functionality, and 
I want to run it on one of my new Linux Synology NAS clones.
There are a bazillion options for replacements and I don't have any 
reliable basis on which to evaluate them.
Do any of you have recommendations for Linux SMTP/POP server software 
that would be suitable for running an internal email system?
By "suitable for running an internal email system" I mean:
1. The system will not/cannot/must not communicate with, or be seen 
from, the internet.
The ClearOS SMTP/POP server is configured to only accept, and send, 
email within its own domain (though this may be achieved by tweaking 
Linux ports rather than directly within the email server, given the 
information provided in the next paragraph, I don't know). The host 
machine is behind a NATing router that does not have any open ports 
for email. I need an email server that can be configured in the same way.
Related to this, I would prefer the ability to configure how the SMTP 
server responds to invalid input, including messages addressed to the 
internet or to invalid internal addresses. The ClearOS server tries 
to send these several times, at increasingly long intervals, so that 
it takes 3 or more days to put out a bounce-back message, and this 
configuration can't be changed. I would rather that bounce-backs take 
place immediately.
2. Must provide all standard POP3 functionality (thank you, but I am 
not looking for an explanation of why I should use IMAP instead), 
including the ability to communicate with a fat Windows email client 
that will sometimes tell it to retain email on the server for a period of time.
3. Free as in beer, preferably, but if not an option, then I would be 
willing to pay a reasonable one-time license fee, not based on the 
number of email accounts, for perpetual use. I am not willing to pay 
for a "software as service" arrangement.
4. Because it is an internal server with no connection to the 
internet, it does not have to have all of the high-paranoia-level 
security features that an internet email server needs. I will cope 
with it if it does not pemit plain text authentication, but plain 
text authentication is perfectly adequate and completely safe for my purposes.
5. It doesn't have to have any built-in spam filtering.
The current CentOS SMTP/POP server has annoying spam filtering 
features that are buggy and can't be turned off. For example, zip 
file attachments (and sometimes docx and xlsx attachments, which are 
just zip files) go missing, and it can choke on other unusual file 
attachments. None of this nonsense is necessary and I would prefer 
for nothing to be built-in that filters spam or messes with 
attachments. If any such thing is built-in, it needs to be able to be 
turned off, and stay off.
6. If it doesn't integrate true listserv capabilities (no fudging 
with "aliases" or numerically-limited forwarding lists) like those of 
MailMan, then it needs to be compatible with MailMan.
7. Traffic requirements: I don't know much about what sort of 
resources a Linux SMTP/POP server requires. Currently we have about 
120 email accounts, each with a 1 GB mailbox on the server, and the 
NAS will accommodate that without even blinking. Conceivably someday 
we could be looking at more like 250 accounts. To the extent to which 
that affects speed/responsiveness, then that's a consideration.
8. Easy to set up and configure, and if not, then EXTREMELY well- and 
reliably-documented.
9. <rant>Probably not an option, but, since I want an integrated 
SMTP/POP server, then, for the sake of the Mother of All that is 
Good, would it be too much to ask for an integrated validation system 
in which, just for laughs, the SMTP server simply compares the 
sender's address to the POP server's list of valid accounts for that 
domain to determine whether or not to accept and relay an incoming 
message, instead of performing bollocks-oriented "test sends" back to 
the user that will fail if the user's inbox, the state of which bears 
no rational relationship to the person's authorization, ability, or 
need, to use the SMTP server to SEND email, is full, and calling it 
"validation", which it is, emphatically, NOT?</rant>
Thanks for all of your suggestions and the benefit of your experience.
Ken Dibble
www.stic-cil.org