We are looking into an email solution for our client. They would like to email their site members once or twice a month. The amount of members is around 6,000. Does anyone know if there is a maximum number of emails that can be sent via ExpressionEngine (Tools > Communicate)? It is set up to send via SMTP (SendGrid). Note: we have the ability to send 40,000 emails per month with SendGrid, so that would not be a bottleneck. I would appreciate any insight or feedback you can offer. I have very limited experience sending emails via the Control Panel's Tools > Communicate.
That doesn't sound like a problem. ExpressionEngine provides 3 protocols; PHP Mail, Sendmail, and SMTP. It's always better to use SMTP where possible and even better if you're outsourcing it to an email gateway like SendGrid.
The only thing to be cautious of is the batch size of large distributions. In the CP go to Admin > Email Configuration and set 'Batch mode' to 'yes'. Then in the field underneath set a value for how many to send out in one batch. It's recommended to start off with something small like 300 or so and see how that goes first. If that goes OK with no errors or timeouts then you can experiment gradually increasing the batch size. SendGrid will be able to provide you with some other useful stats to do with the send success.
The batch mode is there to guard against timeouts and the batches are sent in quick succession so it should still be processed fairly quickly. Other than that it shouldn't have any problems with your scenario. :)