Google Apps mailserver - works fine for < 50 messages; problems when sending more

My asp.net (mvc2) web app webinar registration system sends emails with connection info. Currently we just loop thru the list of attendees and hit a System.Net.Mail.SendMail for each. We send mail via our GoogleApps account (non-premium).

My use case is to send < 50 emails at a time and the system's been working fine. On the occasions where the number pushes a 100 addresses I find error returns with the message:

Service not available, closing transmission开发者_开发知识库 channel. The server response was: 4.7.0 Try again later, closing connection. (MAIL) g31sm4823143ibh.4

Is this message specific to Google's servers (searched on the phrase - seems like 'no'). Is there a recognized 'best practice' for sending emails by code?

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Could it be that gmail just notices that you're sending a lot of messages in a rapid rate indicating it to them you're a spammer (in their eyes)?

If you have hosting I suggest you talk with your provider and see what they can offer you or provide an email address with which you can send more messages, in a controlled way of course.

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GMail definately has a sending limit.

I dont think its widely publicised, but it certainly sounds like you're hitting it.

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Slightly old question for a followup, but to help others landing here, things have changed since then (and they constantly change):

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."

https://support.google.com/a/answer/2956491#sendinglimitsforrelay