As the EMail client, your IP Address can change, but this doesn't matter. The EMail (SMTP) Server's address typically can not change, but now many ISP mail server's IP Address change

. This is one of the reasons we added some features in 1.2 (we're now up to 1.3) that let you deal with a dymamic IP mail server.
First, there is a DNSLOOKUP instruction. So, if your isp mail server name is "mail.myisp.com", use that in your DNSLOOKUP. Then use the DEVWRITE instruction to modify the Do-more's SMTP Server's IP Address that came out from the DNSLOOKUP instruction. Then use the EMAIL instruction. A "SendEMail" PROGRAM code-block using Stages is the best way to perform these 3 operations in sequence (sequencing is hard in pure ladder logic, very easy in Stages).
I know there are examples on how to do this with gmail. This may be your best route, rather than trying to use your ISP's email server, just use a google gmail account instead. People have got that working.