2. You don't use Thunderbird. If it isn't running, or it isn't online, it can't send for you. So you need to use something else.

In both cases it seems to me you need something more than a personal email client.

Zenos said

2. You don't use Thunderbird. If it isn't running, or it isn't online, it can't send for you. So you need to use something else. In both cases it seems to me you need something more than a personal email client.

Zenos,

If Thunderbird won't solve these issues, do you know of any email clients that I can use to solve these two issues?

No. An email client, by definition, is a program installed on an end-user's computer and cannot do anything for you while it is offline.

For what you describe, I can imagine a messy knife-and-fork solution where you export text listings of Sent and incoming folders, parse them for email addresses, then generate a new list of the addresses that are missing from the incoming folder. None of this is something I can see Thunderbird being able to do for you. An automated solution might involve developing a database application, and that would probably move you onto something that has its own smtp client and so eliminates Thunderbird entirely. I think that the Libre Office suite probably offers all the components you need (there is a database, and smtp client) but it will need a significant amount of expertise to string it all together. And of course you could use MS Office, which would almost certainly use Outlook to do the sending part.

To send stuff while you're not online would involve a third party who will do this on your behalf (mailchimp?)

It seems to me that you would end up re-inventing software that already exists, though undoubtedly expensive to purchase. I'm thinking CRM.

https://en.wikipedia.org/wiki/Customer_relationship_management