Customer.io vs GMX Comparison (2026)

What are the differences between Customer.io and GMX?

Forward Email Team
Written by
Forward Email TeamVerified
Published
3/1/26
Time to read
Less than 5 minutes
We recommend Forward Email as the best alternative to Customer.io. We also support webmail: https://mail.forwardemail.net

Email Service Comparison

20 facts in comparison
Customer.iovs.GMX

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.

GMX

Create a free email account fast
"Create a free email account fast"
  • Hardenize Test
  • Internet.nl Site Test
  • Internet.nl Mail Test
  • Mozilla HTTP Observatory Grade
  • Qualys SSL Labs SSL Server Test
$100/moPricing$29.99/yr
No Customer.io does not support storageStorage65 GB
2 MBAttachments100 MB
NoOpen-SourceNo
NoSandboxed Encryption No
NoUnlimited DomainsNo
NoUnlimited AliasesNo
NoTTI No
YesSMTPYes
NoIMAPYes
NoPOP3Yes
YesAPINo
NoE2EE No
NoOpenPGP Yes
NoWKD No

Email Service Screenshots

Closed-source

Customer.io

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

Customer.io is a closed-source email service.
Screenshot by Forward Email
Visit Website
Closed-source

GMX

GMX is a closed-source email service. Their website says: Create a free email account fast

GMX is a closed-source email service.
Photo by Forward Email
Visit Website

Compare Customer.io with 78 email services