Unless you're changing your "FROM:" address (by using several identities) to an email address with a domain name that doesn't match your SMTP domain, no.

Ask your email provider about this.

Just to be sure I understand your reply.... I think you are saying that, if I continue to use the same "FROM:" information as before, some of the risk of my email being categorized as spam could go away if I switch to Thunderbird? [I realize that emails that are truly spammy will get caught by spam filters, no matter what Thunderbird does].

I have contacted my email provider about the spam problem, but they assure me all is well. However, my emails to some friends and customers still go to their spam folders.

Using TB or Outlook is the same with regards to SPAM.

In Thunderbird it's very easy to setup alternate identities. For example I am a member of FSF.org, so I have an alias email address setup as something@member.fsf.org. They don't provide SMTP service, though, so if I configure such an identifiy in Thunderbird and send email through another email server, it will sometimes be categorized as SPAM.

These rules are getting stricter by the day, at more providers. If you write to GMAil recipients your email may not go through or simply end up in SPAM, but writing to someone else may work just fine.

Email from servers notably fails more and more too (for example that "we emailed you a confirmation to open your account").