https://gitlab.com/gitlab-org/omnibus-gitlab/blob/master/doc/settings/smtp.md

编辑/etc/gitlab/gitlab.rb文件以匹配您的SMTP服务器设置和凭据.

Edit the /etc/gitlab/gitlab.rb file to match your SMTP servers settings and credentials.

gitlab_rails['smtp_enable'] = true  
gitlab_rails['smtp_address'] = "smtp.server"  
gitlab_rails['smtp_port'] = 456  
gitlab_rails['smtp_user_name'] = "smtp user"  
gitlab_rails['smtp_password'] = "smtp password"  
gitlab_rails['smtp_domain'] = "example.com"  
gitlab_rails['smtp_authentication'] = "login"  
gitlab_rails['smtp_enable_starttls_auto'] = true
# If your SMTP server does not like the default 'From: gitlab@localhost' you
# can change the 'From' with this setting.
gitlab_rails['gitlab_email_from'] = 'gitlab@example.com'  

然后运行sudo gitlab-ctl reconfigure.