Ubuntu system installs the latest enterprise version of GitLab, but cannot send email without installing postfix,

this is the configuration information. Is there a problem?
what is the relationship between postfix and mail service? can"t you send mail without postfix?

Mar.03,2021
MySQL Query : SELECT * FROM `codeshelper`.`v9_news` WHERE status=99 AND catid='6' ORDER BY rand() LIMIT 5
MySQL Error : Disk full (/tmp/#sql-temptable-64f5-1e3bf83-43336.MAI); waiting for someone to free some space... (errno: 28 "No space left on device")
MySQL Errno : 1021
Message : Disk full (/tmp/#sql-temptable-64f5-1e3bf83-43336.MAI); waiting for someone to free some space... (errno: 28 "No space left on device")
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