ESP or mailbox: which should send your B2B email

Match the path to the message. One-to-one B2B outreach belongs on a real mailbox — Google Workspace or Microsoft 365. Bulk marketing and transactional mail belong on an ESP — Amazon SES, SendGrid, Mailgun, Postmark. Sending either kind down the other path is the cause of a large share of "my deliverability broke" cases.

Why outreach belongs on a mailbox

• Replies have to work. An ESP send is fire-and-forget: the reply lands wherever the Reply-To points, with no thread, no history and no sent-items record in the rep's mailbox. Outreach is a conversation; mailbox sending keeps it in one.
• Filters expect the shape. A plain-text message from a person at a company, via Google's own infrastructure, to one recipient, looks like correspondence. The same text from an ESP's bulk pool with tracking pixels and a footer looks like marketing, and gets scored as marketing.
• Acceptable use. The major ESPs' policies require recipients to have asked for the mail. Cold prospecting lists are not consent-based traffic, and accounts do get suspended for it. That is a business risk, not just a deliverability one.
• Volume per rep is low anyway. A sensible outreach load per mailbox is tens of messages a day, far below any mailbox limit.

Why bulk belongs on an ESP

• Mailbox limits are real and low. Google Workspace allows on the order of 2,000 recipients per day per user, Microsoft 365 around 10,000 recipients per day with a per-minute rate cap and a per-message recipient cap. You will hit these, and the failure mode is a hard rejection mid-campaign.
• You need unsubscribe handling, suppression and bounce processing as infrastructure, not as something your sales tool half-implements.
• You need per-stream separation. A password reset must not share reputation with a promotional blast. Postmark enforces this with message streams; on SES you achieve it with separate configuration sets and subdomains.

The decision table

The mistakes this prevents

• Blasting a cold list through SES or SendGrid. The bounce and complaint rates from cold data will cross SES's 5% bounce and 0.1% complaint review thresholds quickly, and the account review is not a fast process.
• Running a newsletter from a sales mailbox. You will hit the daily recipient cap, and the rejection string (550 5.4.5 Daily user sending limit exceeded on Google, rate limits on Microsoft) arrives after some recipients have already been sent.
• Mixing transactional and marketing on one subdomain. When the marketing stream attracts complaints, password resets start landing in Junk, and that is the outage nobody connects to the campaign calendar.
• Letting the sales tool send on your behalf without alignment. If a tool sends via its own infrastructure using your From domain, check DMARC alignment: SPF can pass for the tool's domain while DMARC fails because the visible From does not align.

If you need both, separate the domains

Run corporate mail on example.com, outreach on a separate domain or a dedicated subdomain, and bulk marketing on another subdomain. Each gets its own DKIM selector and its own reputation. The cost is a few DNS records; the benefit is that a bad month in one stream cannot take down your invoices or your recruiting mail.

Publish DMARC on the organisational domain with a policy you intend to keep, and remember subdomain policy inheritance: sp= controls subdomains, so a strict p=reject with no sp= applies to the new subdomain too, which is usually what you want but will break a tool you forgot to authenticate.

What not to do

• Do not buy a dedicated IP to solve an outreach problem. Outreach volume is far too low to keep a dedicated IP warm, and a cold dedicated IP is worse than a well-run shared pool.
• Do not rotate through dozens of throwaway domains instead of warming a few properly. Receivers score domain age and consistency; churn is itself a signal.
• Do not put link tracking on 1:1 outreach. A redirect domain with no reputation, wrapping every link, is one of the cheapest ways to look like bulk mail.
• Do not assume one tool can own both paths. The sending mechanics genuinely differ.

MailPilot supports both paths deliberately: OAuth connections for Gmail, Google Workspace, Outlook and Office 365 mailboxes, and SMTP or IMAP for SendGrid, Amazon SES, Mailgun, Postmark and custom SMTP. The same warm-up, SPF/DKIM/DMARC checks, per-provider placement measurement and daily reputation tracking apply whichever side your mail leaves from, and every plan includes unlimited mailboxes, so splitting streams across more domains does not cost more.