...$ man 5 postconf | grep size
Переформатирование postconf(5), подождите...
berkeley_db_create_buffer_size (default: 16777216)
The per-table I/O buffer size for programs that create Berkeley DB hash
berkeley_db_read_buffer_size (default: 131072)
The per-table I/O buffer size for programs that read Berkeley DB hash
body_checks_size_limit (default: 51200)
bounce_size_limit (default: 50000)
header_size_limit (default: 102400)
mailbox_size_limit (default: 51200000)
The maximal size of any local(8) individual mailbox or maildir file, or
zero (no limit). In fact, this limits the size of any file that is
This limit must not be smaller than the message size limit.
message_size_limit (default: 10240000)
The maximal size in bytes of a message, including envelope information.
manager, and the maximal size of the size of the short-term, in-memory
the amount of free space is less than 1.5*$message_size_limit. To
value that is at least 1.5*$message_size_limit.
The maximal size in bytes of an individual mailbox or maildir file, or
По умолчанию - 10 мегов, возможно другой сервер ограничивает. /etc/postfix/main.cf смотри