In order to be able to send messages on your behalf from Mailkit servers, it is necessary for them to be authorized in the SPF domain record by adding our SPFs to your record. In your domain's DNS settings, look for a TXT record starting with "v = spf1" and add "...include: spf.mailkit.eu ..." to this record.
If your domain doesn't have an SPF record, we recommend that you create such a record in the form of "v=spf1 mx include:spf.mailkit.eu ~all"
CAUTION: Each domain can contain only ONE SPF record! It is therefore not permissible for there to be more than one SPF record, but all permissions must be in one SPF record. This record must not exceed the limit of 10 lookups, so it is necessary to monitor its length. Of course, this has not only practical reasons but also security. Therefore, be careful what is allowed in your domain and whether your SPF records are completely valid. We recommend using SPF analysis on the Dmarcian.com for checking.