There are plenty of cloud storage providers that support the email SMTP hosting.
Unlike AWS S3, I highly recommend AWS SES.
- Amazon Simple Email Service (SES)
- Mailgun
- SparkPost (Known to cancel Mastodon instances accounts for "spam")
- https://www.zoho.com/zeptomail/
- https://www.smtp2go.com/ (recommended by pettingzoo)\