Email Detective — ESP Tracker

Which ESPs deliver slowly

How often each ESP's mail arrives late, measured from the sending server to the inbox (parsed from message headers). Ranked within each group by the share taking 5+ minutes — split by the kind of customer each platform serves, so big-sender tools aren't compared against small-business ones.

Delays usually mean the receiver is throttling or greylisting the sender.

Enterprise platforms

Built for large senders — Salesforce Marketing Cloud, Braze, Iterable, Adobe Campaign, and similar.

ESP	Emails	Delay mix	Slow	Very slow	Severe
Sailthru	1,005		14%	14%	0
Salesforce MC	4,839		11%	9.8%	0.4%
Adobe Campaign	867		1.8%	2.3%	0
Marketo	1,004		0.5%	0.9%	0.2%
Salesforce CRM	1,141		1.2%	1.0%	0
Bloomreach	1,010		1.5%	0.8%	0
Iterable	5,129		1.1%	0.5%	0
Braze	2,449		0.2%	0.3%	<0.1%

SMB platforms

Aimed at small and mid-size senders — Mailchimp, Klaviyo, Brevo, HubSpot, and similar.

ESP	Emails	Delay mix	Slow	Very slow	Severe
Mailchimp	2,819		11%	4.6%	0.1%
ActiveCampaign	1,383		1.2%	4.2%	0.4%
Beehiiv	3,861		3.8%	2.6%	0.1%
Omnisend	895		3.0%	1.7%	0.2%
Shopify Messaging	1,114		0.3%	1.3%	0.4%
Seguno	1,065		0.8%	1.2%	0.4%
Customer.io	1,706		4.9%	1.1%	0
MailerLite	666		4.8%	0.9%	0
Klaviyo	7,138		2.0%	0.6%	<0.1%
Intercom	676		0.4%	0.6%	0
Substack	1,474		0.2%	0.5%	0
HubSpot	3,677		3.5%	0.4%	0
Brevo	1,486		0.4%	0.2%	0
Kit	2,064		0.7%	0.2%	0

Transactional / developer

Send-API platforms used by senders of every size — SendGrid, Mailgun, Postmark, Amazon SES, and similar.

ESP	Emails	Delay mix	Slow	Very slow	Severe
SparkPost	2,727		1.7%	3.1%	0.1%
Mailgun	5,691		2.1%	2.3%	0.1%
SendGrid	14,873		1.5%	0.7%	<0.1%
Postmark	1,117		0.9%	0.4%	0
Amazon SES	15,939		0.6%	0.1%	<0.1%

Based on 96,399 delivery-timed emails from the last 90 days (ESPs with 500+ shown). Delay is attributed to each brand's primary ESP, so read it as "mail sent via X arrives slowly this often." Severe (30 min+) is tracked from today onward; older 30-min delays are still counted under very slow.