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First
Published: September 20, 2006
Minor Update: October 31, 2006
There are many limits in effect from
various sources when sending email. This article illustrates the aspect
of size limits. Besides
size limit, email sending is also subject to content filtering, per-hour
and per-day throttling, destination/source blacklists, etc.
Introduction
It is absolutely necessary to enforce email
size limits at various places. There are numerous practical, technical,
legal and ethical reasons, beyond our scope of discussion here. Suffice
to state that: Without such limitations, worldwide meltdown of The
Internet (not just email) would be a daily occurrence. Email is by
nature insecure, unaccountable, infinitely recursive, and most
significantly―can be
leveraged
exponentially! Any slightest misstep, be it inadvertent or malicious
(both inevitable) could result in catastrophic collapses.
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Various
Stages of Size Limits An Email Message Is Subject To |
| Sender Email Client (e.g. Outlook) |
Typically unlimited (except by disk space) |
| Sender Exchange Server* |
Per site administrator policies |
| Sender SMTP Server per message |
RR: 5M, Frontier: 10M, Mach-4: 7M,
Local: ??? |
| Destination POP Server Quota** |
Typically no per-message limits, only quota in effect:
RR: 10M, Frontier: 25M
Mach-4: Per hosting plan: Site quota & User quota |
| Destination Exchange Server* |
Per site administrator policies |
| Recipient Email Client (e.g. Outlook) |
Typically unlimited (except by disk space) |
Space Usage for Typical Scenarios
|
Quota |
Text email
messages (4K) |
High Resolution
Photo (250K) |
128Kbps Song
/PDF doc (4M) |
| 5M |
1,250 |
20 |
1.25 |
| 10M |
2,500 |
40 |
2.50 |
| 15M |
3,750 |
60 |
3.75 |
| 20M |
5,000 |
80 |
5.00 |
As you can see from the table above, a 10M
quota is enough to hold 2,500 email messages. That's 1 full year's worth
for the typical user. That same quota will also hold 40 high resolution
pictures, or just 2 typical songs or PDF documents.
Due to extremely poor efficiency of
the antiquated MIME protocol used to encode attachments into plain text,
the resulting email message is typically 1.2x to 1.5x the size of the
original attachments. e.g. A 5M message can hold only 3.3M~4.2M worth of
file(s)!
Email server is a "holding tank" or buffer.
A user requires enough space quota to hold
all the email arriving during the interval between retrievals (typically 5 minutes to 24 hours). For those saving messages on server
for retrieval at alternate locations,
space is required for all messages received within the retention period
(typically 5 to 14 days). Finally, those cumulating message on server
permanently and rely on it as primary storage must provide space for the entire set
of messages and attachments.
To summarize, there are 3 modes of
operations, as related to email server space:
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Retrieve and Remove (Standard)
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Retrieve and Deferred Removal
(Multi-Station)
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Retrieve and Retain (Host-based)
For those with hosting plan, it's customary to over-allocate
within the organization,
anticipating that most users would not actually use up their
quotas. For
example: 100M total space is obtained, and 10M quota is allocated to each
of the 12 users, resulting in a theoretical maximum usage of 120M. It is
possible for site quota to be exceeded, even when the user
quotas are still within limits.
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Solutions To Sidestep
Size Limits:
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Compress the attachment
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Send multiple attachments in separate messages
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Split attachment into multiple files
(requires joining at recipient
end)
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Implement and use your own in-house or datacenter-hosted
SMTP
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Upgrade residential ISP line to commercial (avoids port
25 filtering)
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Upgrade hosting plan
(increase quota)
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Re-allocate user quotas within your hosting plan
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More frequent mail retrieval
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Reduce message retention period on server
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Convert from POP-based to client-/Exchange-based email
storage
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Use webmail:
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Avoid attachments & use proper alternate file transfer
methods
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SPAM management
(conserve quota for legitimate
email)
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*Exchange Server (if
installed & used) could impose limits per policies set by site
administrator. It is not
applicable for small offices that do not have it.
**Quota can be exceeded if
user is not checking frequently enough, or opt to save message on
server. POP server is not applicable for sites directly delivering to
their Exchange Servers.
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