Guidance on sending emails to many subusers (100+)

[arkitex]

Issue Summary

I'm trying to use this library to send emails on behalf of many subusers - in the hundreds, if not thousands. It appears that the primary pattern is to use new Sendgrid(apiKey) in order to send - but that creates a FixedThreadPool with 8 threads. If I wish to send emails to even 10 subusers, this means I'll have 10 FixedThreadPools with 8 threads each - not desirable.

Is there any guidance on how to appropriately use this library for the above use-case? It looks like we could implemented SendgridAPI, but I'm hoping to avoid that if possible.

I'd use on-behalf-of, but that is not supported for mail.send.

Technical details:

• sendgrid-java version: 4.4.8