Customer.io vs Startmail Comparison (2025)

What are the differences between Customer.io and Startmail?

Forward Email Team
Written by
Forward Email TeamVerified
Published
7/1/25
Time to read
Less than 5 minutes
We recommend Forward Email as the best alternative to Customer.io.

Email Service Comparison

20 facts in comparison
Customer.iovs.Startmail

Customer.io

Send messages people actually want to receive using real-time customer data
"Send messages people actually want to receive using real-time customer data"
  • Hardenize Test
  • Internet.nl Site Test
  • Internet.nl Mail Test
  • Mozilla HTTP Observatory Grade
  • Qualys SSL Labs SSL Server Test
vs.

Startmail

Private email you can trust
"Private email you can trust"
  • Hardenize Test
  • Internet.nl Site Test
  • Internet.nl Mail Test
  • Mozilla HTTP Observatory Grade
  • Qualys SSL Labs SSL Server Test
$100/moPricing$7/mo
No Customer.io does not support storageStorage20 GB
2 MBAttachments25 MB
NoOpen-SourceNo
NoSandboxed Encryption ❌ Startmail claims to use a User Vault, but this statement has not been audited and their source code is closed-source
NoUnlimited DomainsNo
NoUnlimited AliasesNo
NoTTI No
YesSMTPYes
NoIMAPYes
NoPOP3Yes
YesAPINo
NoE2EE ⚠️ Supported if both sender and recipient are using OpenPGP
NoOpenPGP ⚠️ Supports OpenPGP, but does not support WKD
NoWKD No

Email Service Screenshots

Closed-source

Customer.io

Customer.io is a closed-source email service. Their website says: Send messages people actually want to receive using real-time customer data

Customer.io is a closed-source email service.
Photo by Forward Email
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Closed-source

Startmail

Startmail is a closed-source email service. According to their website: Private email you can trust

Startmail is a closed-source email service.
Screenshot by Forward Email
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Compare Customer.io with 78 email services