Batches and ceilings
Settings → Email → Sending limits controls the pace. Campaigns go out in batches (Recipients per batch) with a pause between them, under an optional hard ceiling per hour across every campaign running at once. Invoices, sign-in links, documents, and questionnaires are never throttled — only campaign and sequence mail. A send that is paused by a limit resumes by itself; nobody is dropped.
Warm-up for a new domain
A domain with no sending history is the one providers trust least, and putting a whole list through it on day one is the fastest way to get it filtered. With "Ease a new sending domain in gradually" on, a newly verified domain sends at most the daily caps you set — 50, 100, 250, 500, 1,000 by default — for its first days, and the rest of a campaign goes out as the cap lifts. The ramp is per domain: adding a bulk subdomain does not slow the domain your invoices use, and a domain you have been sending from for longer than the schedule is never affected. A campaign shows which day of warm-up it is on and today's cap, so a paused send is explained rather than mysterious.
The circuit breaker
Every campaign watches its own results as it sends. Once enough messages have gone out to judge (Check after — 500 by default), the send stops itself if bounces exceed the bounce limit (5% by default) or spam complaints exceed the complaint limit (0.3% by default — Gmail's own threshold, above which delivery degrades for every future send). A stopped campaign says why on its results page. Fix the list, then resume.
Deferred is not failed
A recipient held back by a limit — the hourly ceiling, the warm-up cap, or a provider asking us to slow down — is deferred and sent later. A recipient whose address bounced is failed and is not retried. The results page keeps the two apart, so a paused send never reads as a broken one.