这种服务安利一下 Google Suite 收费版，有 SMTP Relay 服务，直接秒发。。。限制是一天最多 1w 封，用户少的话，按照用户数量的 130 倍
如果你用 GAE ，那直接内网，延迟是 1 ms 左右
https://support.google.com/a/answer/2956491?hl=enLimits per user
A registered G Suite user can't relay more than 10,000 messages in a 24-hour period, and can't relay messages to more than 10,000 unique recipients per 24-hour period. Users exceeding either of these limits see the error "550 5.4.5 Daily SMTP relay limit exceeded for user."
The message count is based on the address of the envelope sender presented during the SMTP relay transaction. If the envelope sender is not a registered user, then the per-user limits don't apply. Addresses in the From: and Reply-to: fields are not considered. Nor do we consider the address presented during SMTP authentication, which is turned on if you select the Require SMTP Authentication option, described below.
Any sender, whether or not they're a registered user,can also be prevented from sending messages if a customer relay limit has been reached. In this case, they see a different error message based on limits per customer.
Limits per customer
These limits are determined by the number of user licenses in your G Suite account. For small customers, these limits come into effect much earlier than the limits per user.
There are two per-customer limits:
The maximum number of total recipients allowed per customer per 24-hour period is approximately 130 times the number of user licenses in your G Suite account, with an upper bound of 4,600,000 recipients per 24-hour period for large customers. If a customer exceeds this limit, users see the error "550 5.7.1 Daily SMTP relay limit exceeded for customer."
The maximum number of total recipients allowed per customer in a 10-minute window is approximately 9 times the number of user licenses in your G Suite account, with an upper bound of 319,444 recipients per 10-minute window for large customers. If a customer exceeds this limit, users see the error "450 4.2.1 Peak SMTP relay limit exceeded for customer."