Marking incoming emails as SPAM isn´t by far not only problem of program, which sends emails. During the way the message goes through by some spam filters too (in recipient email client, on server with box of recipient etc.) and every from them can have setup other criteria for evaluation.

It depends largely on it, how the message appears and mainly what includes. If email includes for example too much graphics, big headlines, many words as "buy", "the best offer" etc., it´s very suppossed to be marked as SPAM.

Program SendMails includes of course some advanced functions so that it reduces this risk (e.g.special content and coding email, sending of every email individually, possibility to send emails only in text form, graphics sending together with message - no displayed from external resource etc.), but much more it depends on content of messages.

Recommendation:
- smaller quantity of graphics
- any extreme formatting of text
- avoid of "dangerous" words
- don´t send emails in copy (hidden too) - if quantity of emails enables it
- to limit number of attachments

If do you want to have fast 100% guarantee, your email will not be marked as SPAM, send your emails rather in plain text than in HTML. This email doesn´t appear of course so nice and needn´t be for addressing of customers so right...

Other thing is, if recipient himself marked your messages as SPAM, it´s not possible to make anything with it after that. The one possibility is then sending of messages under other email address of sender ;)