In order to warm up an email alias, you would need to connect the primary email address of that alias to the platform since this is where all email activity actually originates from.

An alias only essentially changes the displayed email address of incoming mail to a recipient but all other activity/email headers will display the primary email address that actually carried out that email send.

All email activity under an alias occurs through the main email address it's associated with, so if an alias such as hello@domain1.com is connected to another email like info@domain.com, all email activity occurs through info@domain.com in that situation since hello@domain1.com wouldn't have it's own sending activity directly.

Generally, we wouldn't suggest using email aliases at all for outreach since they generally cause more issues than simply getting that specific email address set up on its own.

Email aliases work by replacing the presented sender address with that of the alias in an email message so that it masks the original sender address performing this email activity, but since an alias doesn't actually perform any email activity itself, the header of the outgoing emails still would be that of the primary email address the alias is tied with, so it's no different than performing email activity directly from the primary email address.

Also for deliverability with aliases, overall they typically do perform worse than just sending out an email directly from an email address since some spam filters/mail servers will be a bit more cautious in regards to accepting incoming mail where the sent-from address is changed.