postfix queue file write error message size limit

If you get an error like:
Aug 21 12:41:03 webb4 postfix/sendmail[11234]: fatal: web1_user(12967): queue file write error

in your mail log, then the vale for message_size_limit in postfix main.cf is too low. Edit postfix main.cf:

vi /etc/postfix/main.cf
and set the message size limit to a higher value. E.g.
message_size_limit = 900000000
and restart postfix:

/etc/init.d/postfix restart

If there is no message_size_limit set in main.cf, just add the line.

The configuration option is "message_size_limit" and by default is set to 10240000 bytes which is roughly 10MB. You can see what setting your postfix install is currently using by issuing the following command:

postconf | grep message_size_limit

Just doing "postconf" on its own will show all the options so using grep on "message_size_limit" filters the output to just the setting we want. For the default setting, you'll see this:
message_size_limit = 10240000
To change the setting to some other value, open the main.cf file in your favourite text editor (it's often at a location like /etc/postfix/main.cf) and either edit or add the following line, depending if it's already in the file or not.
message_size_limit = 20480000
In the above example I've changed it to ~20MB.
Then reload postfix like so:
service postfix reload

Recommended:

postconf | grep message_size_limit
postconf -e message_size_limit=20480000             
service postfix reload

Something to note: many emails use base64 (there is 8bitmime but support is far from universal). So the space taken up by the attatchment in the email will be about a third bigger than the total size of the attatchments. Then you have to consider the rest of the mail.
So if you want to receive 20MB files you need to set the message size limit to around 27MB.