imapsync/FAQ.d/FAQ.Messages_Too_Big.txt

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$Id: FAQ.Messages_Too_Big.txt,v 1.8 2021/12/16 11:44:26 gilles Exp gilles $

This document is also available online at
https://imapsync.lamiral.info/FAQ.d/
https://imapsync.lamiral.info/FAQ.d/FAQ.Messages_Too_Big.txt

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              Dealing with too big messages with Imapsync.
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Questions answered in this FAQ are:

Q. Imapsync reports errors like these ones:
   "could not append...Maximum size of appendable message has been exceeded"
   "could not append...maximum message size exceeded"
   "The most frequent error is ERR_APPEND_SIZE"
   What does it mean?

Q. Does imapsync limits big messages?

Q. What are the well known servers messages size limits?

Q. What can I do to transfer messages bigger than what allows the 
   imap destination server?

Q. The solution --truncmess xxxx doesn't work with Gmail. 
   What can I do?


Now the questions again with their answers.

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Q. Imapsync reports errors like these ones:
   "could not append...Maximum size of appendable message has been exceeded"
   "could not append...maximum message size exceeded"
   "The most frequent error is ERR_APPEND_SIZE"
   What does it mean?

R. It means the message couldn't be transferred to the destination
   account because it is too big, imapsync tries to APPEND it to the
   server (that's IMAP jargon) but the server refuses it by saying
   either "NO. Maximum size of appendable message has been exceeded"
   or "NO. maximum message size exceeded".


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Q. Does imapsync limits big messages?

R. No. Imapsync synchronizes messages no matter their sizes.

When a message is copied to the destination server, the destination
server accepts it or not. It does it as a whole, it takes the full
message or nothing. The destination server can refuse a too big
message, it then triggers an APPEND error message in imapsync.

So far, December 2021, the biggest message transferred by imapsync on
the online /X service is 1.51 GiB. Who can top that?


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Q. What are the well known servers messages size limits?

   * Gmail message size limit is 35 MB. 
     Gmail announces APPENDLIMIT=35651584 (June 2021).
   
   * Office365 message size limit is 45 MB by default
     and it can be upped by configuration. See 
     https://imapsync.lamiral.info/FAQ.d/FAQ.Office365.txt
     Q. I see "NO Maximum size of appendable message has been exceeded"

   * Exchange message size limit is 10 MB by default
     and it can be upped by configuration. See
     https://imapsync.lamiral.info/FAQ.d/FAQ.Exchange.txt
     Q. I see "NO Maximum size of appendable message has been exceeded"

   * Dovecot message size limit is variable. It is defined
     by the sysadmin. For example, quota_max_mail_size = 100M
     https://doc.dovecot.org/configuration_manual/quota/
     
   * Usually, any IMAP server has a message size limit, defined 
     by the sysadmin. You discover it sooner or later

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Q. What can I do to transfer messages bigger than what allows the 
   imap destination server?

R1. A solution is ask imapsync to truncate the message to the maximum
size allowed before copying it to host2 by using the option
--truncmess.

That's a little dirty but it's ok if you prefer having a truncated
message than no message at all. The attachments after the truncation
won't be available but everything before should be ok. It may also
depend on the software tool reading the email message.


To truncate messages to 10 MB, use: 

  imapsync ... --truncmess 10_000_000


R2. A not-so-dirty solution would be to transform the big messages,
replace the attachments with links to the attachments stored
elsewhere.  Well, it's not done and it's too much complicated compared
to a simpler solution which is to allow bigger messages on the
destination account.

As time goes on, disk space is cheaper and cheaper.  Don't make the
new user account, the destination account, worst than his previous
account: buy disks.

Monitor and rule your hardware resources, serve your users,
not the contrary.


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Q. The solution --truncmess xxxx doesn't work with Gmail. 
   What can I do?

R. Gmail currently limits messages size up to 35651584 bytes, aka 34
  MiB (June 2021) but imapsync knows it so it avoid to transfer these
  big messages. Solution, use:

  imapsync ... --truncmess 35_651_584 --appendlimit 1_000_000_000_000

Explanation. By default, imapsync checks if the host2 imap server
announces a message size limit with "APPENDLIMIT=xxxx" in its
CAPABILITY response, like Gmail does.  Imapsync takes this APPENDLIMIT
value to set automatically the --maxsize option, unless --maxsize is
already set on the command line and has a smaller value than the
APPENDLIMIT value.

1_000_000_000_000 is one TB, I guess you don't have any message of
that size yet.

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