Sympa receives incoming messages in its msg spool. An image of the message is then created (a MIME::Entity, using MIME-Tools CPAN library) for later processing. At the end, the MIME::Entity object is dumped to the mail tranfer agent (sendmail, postfix,...). Note that the message might be altered by this intermediate MIME::Entity object (row size for Base64 encoded MIME parts for example). The use of the MIME::Entity intermediate object is skipped is a message is S/MIME signed because any changes in any part of the message body would break the S/MIME signature.

Note also that Sympa might perform the following changes to the distributed messages; however some of this alterations of the messages can be configured.

• SMTP header fields are added/removed for loop prevention (see loop_detection), for RFC 2369 compliance (see rfc2369_header_fields) or through customizations (see remove_headers).
• The subject of the message might be changed to add a custom subject (see custom_subject).
• Headers and footers may be added. They are either added as separate MIME parts or within the message body, if it is of text type (see message_header_and_footer)
• Message content-type may be changed from multipart/alternative to text/plain or text/html if the user has selected the corresponding reception mode (see multipartalternative).

Sympa distribution process copes well with MIME messages, including those including attachments. However there are situations special processing for messages with attachments:

• max_size : this parameter controls the maximum allowed size for a distributed message,
• urlize : list members can select this reception mode to have attachments removed from the message and stored on the list server. Urlizization depends on the message size and the urlize_min_size parameter.

Sympa allows to send customized messages to mailing lists ; each subscriber will receive a customized version of the initial message. The initial message is processed as a template with embedded variables and is parsed, for each subscriber, with the attributes Sympa has associated to her. Note that you can freely create any subscriber attribute, see the custom attributes feature.

The personalization of a message consists to replace variables by the attributes of the user. It uses the TT2 tags in order to distinguish them from text: [% %].
The personalization process occurs as follows:

1. Sympa get the message containing the TT2 variables,
2. it retrieves the user data in the database,
3. it replaces the variables by the found data.

Here is the syntax for writing a variable: [% user.custom_attribute.ID.value %]
ID is the custom attribute id sub-parameter

Here are the steps to send a customized message to a list :

1. this feature is enabled / disabled using a list parameter: merge_feature. If set to on, messages will be merged.
   
2. if you choose defined custom users attributes, they can be used to personalize messages.
3. users fill their personal user attributes for the list themselves. (name, surname, civility, ...)
   
4. the message can be composed either from Sympa web interface or using your favourite mail client. Simply insert the variables in this message in the form of a TT2 tags. (ex: [% user.custom_attribute.civility.value %] to refer to the subscriber's civility).
5. Here is a diagram to better understand the process of customization
   

Note :
This feature is likely to have an influence on the message dissemination performances. Indeed, to send personalized messages to each user, Sympa opens an SMTP session for each recipient. Previously, it tried to open a session for several recipients if they belonged to the same domain.