以下是一个使用Python编写的Linux监控服务器磁盘、内存空间使用率并在达到90%时发送邮件的脚本：

import os
import smtplib
from email.mime.text import MIMEText
from email.header import Header

# 设置阈值
DISK_THRESHOLD = 90
MEMORY_THRESHOLD = 90

# 获取磁盘使用率
def get_disk_usage():
    disk = os.statvfs("/")
    total = disk.f_blocks * disk.f_frsize
    used = (disk.f_blocks - disk.f_bfree) * disk.f_frsize
    usage = int(used / total * 100)
    return usage

# 获取内存使用率
def get_memory_usage():
    with open('/proc/meminfo', 'r') as mem:
        free_memory = 0
        total_memory = 0
        for i in mem:
            sline = i.split()
            if str(sline[0]) == 'MemTotal:':
                total_memory = int(sline[1])
            elif str(sline[0]) in ('MemFree:', 'Buffers:', 'Cached:'):
                free_memory += int(sline[1])
        used_memory = total_memory - free_memory
        usage = int(used_memory / total_memory * 100)
        return usage

# 发送邮件
def send_email(subject, content):
    sender = 'your_email@example.com'
    receivers = ['receiver_email@example.com']
    message = MIMEText(content, 'plain', 'utf-8')
    message['From'] = Header("Server Monitor", 'utf-8')
    message['To'] = Header("Admin", 'utf-8')
    message['Subject'] = Header(subject, 'utf-8')

    try:
        smtpObj = smtplib.SMTP('localhost')
        smtpObj.sendmail(sender, receivers, message.as_string())
        print("邮件发送成功")
    except smtplib.SMTPException:
        print("Error: 无法发送邮件")

# 主函数
def main():
    disk_usage = get_disk_usage()
    memory_usage = get_memory_usage()

    if disk_usage >= DISK_THRESHOLD or memory_usage >= MEMORY_THRESHOLD:
        subject = "服务器资源告警"
        content = f"磁盘使用率：{disk_usage}%，内存使用率：{memory_usage}%"
        send_email(subject, content)

if __name__ == '__main__':
    main()

请将your_email@example.com替换为您的发件人邮箱地址，将receiver_email@example.com替换为收件人邮箱地址。此外，您需要配置本地SMTP服务器以使邮件发送功能正常工作。如果您使用的是Gmail，可以参考这个教程来配置SMTP服务器。