One of the useful tools to resolve this issue would be e-mail headers. Often they can reveal WHY that e-mail went to spam folder. E-mail headers contain full information on what happened to e-mail while it was in transit. Often you will be able to see results of anti-spam processing which was done on the e-mail. And often it is full report which can tell you why this e-mail was considered as spam by that software.
In this example we can see header
X-Spam-Status: which set to
score=3.7. This header is just information that anti spam software analyzed the e-mail and based on the settings it has made the decision that this e-mail has
spam score 3.7 It is semi-arbitrary scale and amount of spam points which e-mail will get depends on the software.
Here is another example:
X-Spam-Status: Yes, hits=27.3 required=5.0
Here we can see that e-mail was evaluated to have 27.3 spam score (
hits=27.3). This software was configured to consider any e-mail with score more than 5 to be spam (
required=5.0). So what it does in this case? It sets flag
YES. This is just evaluation. Now you can set filter on your e-mail client which will put any e-mail with
X-Spam-Flag: YES to the spam folder.
I just gave you one example of how e-mail headers work. Probably you will be able to see reason why your e-mails are considered to be spam.
Ask people who receive your e-mails to send you back e-mail headers for research. Every modern e-mail client should be able to get those.
I am not promising it will work for you. But e-mail headers and server logs - are first things you should look at while resolving e-mail issues.