Many of the large email providers (Gmail, Yahoo, Office 365) are getting picky about accepting email that has no 'validation' mechanisms. Even if your email is not spam or junk they may treat it as so and either reject the message or put it in the recipients junk folder.
Whilst we don't provide DMARK or DKIM features on our email service customers are are welcome to use their own email servers and configure them as they wish.
However, customers may be able to add a SPF record to their domain to help get their messages through. In short, an SPF record says which email servers are allowed to send messages for you.
Typically, our customers would use smtp.aa.net.uk to send email - however thought must be given to other servers that may also send email from your domain - eg you may have a 3rd party web server or some other service that sends email on our behalf - these would want to be included too.
To cover the IP addresses of the A&A email servers, you can include
v=spf1 mx include:_spf_include.aa.net.uk ?all
Testing with Gmail
You can send an email to a gmail address, and then look at the received headers to see what Gmail think about the SPF records. (In gmail, open the email, click the 3 dots and select show original)
ARC-Authentication-Results: i=1; mx.google.com; spf=neutral (google.com: 184.108.40.206 is neither permitted nor denied by best guess record for domain of firstname.lastname@example.org) email@example.com
ARC-Authentication-Results: i=1; mx.google.com; spf=pass (google.com: domain of firstname.lastname@example.org designates 220.127.116.11 as permitted sender) email@example.com