Any suggestions for integrating MODX with a third-party email system or otherwise generate email newsletters from within MODX?

I would be grateful for any suggestions/experience you could share. Thanks!

I've been very happy with the perl DADA mailing script. It's easy to embed it in the Manager, either as a module with Evo or a CMP with Evo. And you can specify the URL to use for the newsletter, making it easy to use a MODx resource, shifting the current one to an archive simply by changing the alias, and giving the alias used in the DADA configuration to a new resource for a new newsletter.

Campaign Monitor is a third party newsletter with easy to implement sign up forms. It is a paid service

Have used CampaignMonitor and integrated subscription forms in the past and find it very good. However, this client wants to be able to use MODX to create a resource and then "copy & paste" the HTML code from it into an email etc etc. I keep advising him that he would really be better off with an external system (reports, analytics, avoid blacklisting etc.) but he wants me to explore this option anyway ...

Eek unless that client is very experienced writing markup for emails there will likely be a ton of formatting errors

=))...had that "scenario" a few weeks ago with a client. I solved it with a eMail template (you could start with the base from mailchimp for example...quite good support in mailclients I guess =D) which is the newsletter and I made it possible via MIGX to add a variable amount of content (ie picture and text aside, but could be whatever you want...), so I have a container "Newsletter" and the sub-resources are the single Newsletters which can then be edited via the migx grid and have the eMail Template. You get the HTML by previewing the resource and viewing its source...copy paste wherever you want =)...

In my case this was just a really fast solution because GroupEletters (Addon) was not working as expected and I had to find a fast way to "export" (= copy'n'paste) the newsletters to mailchimp.

DADA can be given a URL to a page to use as its email content, so the customized template works very well with it. Some hosting companies recommend using it, since it has several options for minimizing its impact on the server.